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Merge pull request #6 from rust-console/lokathor

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version 0.2 canidate: keyinput now supported
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57
README.md
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@ -6,59 +6,10 @@ A crate that helps you make GBA games
# First Time Setup
[ketsuban](https://github.com/ketsuban) is the wizard who explained to me how to
do this stuff.
1) Install `devkitpro`. They have a graphical installer for Windows, or you can
use pacman or whatever for linux things I guess. The goal here, among other
things, is to have a `binutils` setup that's targeting `arm-none-eabi`. We'll
also use some of their tools that are specific to GBA development so if you
for some reason already have the appropriate `binutils` then you probably
still want devkitpro.
* On Windows you'll want something like `C:\devkitpro\devkitARM\bin` and
`C:\devkitpro\tools\bin` to be added to your PATH. I'm not sure on the
directories for other systems. If you know then file a PR with the info.
2) Next we use `cargo install cargo-xbuild` to get that all setup.
3) Create a binary project. We're going to want nightly rust for this, so if you
don't already have it set to default to nightly you should [set that
up](https://github.com/rust-lang-nursery/rustup.rs#the-toolchain-file) for
this project.
4) Clone this repo. It has an appropriate `main.rs` that will draw three test
dots as well as other support files:
* crt0.s
* linker.ld
* thumbv4-none-agb.json
* build.rs
5) Run `arm-none-eabi-as crt0.s -o crt0.o` to build the `crt0.s` into a `crt0.o`
file. You could theoretically to it only when `crt0.s` changes, but in out
`build.bat` file it's set to simply run every single time because it's a
cheap enough operation.
6) Build with `cargo xbuild --target thumbv4-none-agb.json`
* The file extension is significant, and `cargo xbuild` takes it as a flag to
compile dependencies with the same sysroot, so you can include crates
normally. Well, crates that can run inside a GBA at least (Which means they
have to be `no_std`, and even `no_alloc`).
* This generates an ELF binary that some emulators can run directly (which is
helpful because it has debug symbols).
7) Also you can patch up the output to be a "real" ROM file:
* `arm-none-eabi-objcopy -O binary target/thumbv4-none-agb/debug/gbatest target/output.gba`
* `gbafix target/output.gba`
8) Alternately, you can use the provided `build.bat` file (or write a similar
`build.sh` file of course), which does all four steps above.
9) Time to read the [Tonc](https://www.coranac.com/tonc/text/toc.htm) tutorial
and convert all the C code you see into rust code.
* Also of great importance and help is the
[gbatek](https://problemkaputt.de/gbatek.htm) document.
* In time we'll have our own book that's specific to rust and this crate and
all that, but until then you're gonna have to look at a bunch of C code.
This crate requires a fair amount of special setup. All of the steps are
detailed for you [in the 0th chapter of the
book](https://rust-console.github.io/gba/ch0/index.html) that goes with this
crate.
# Contribution

4
book/src/SUMMARY.md
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@ -9,3 +9,7 @@
* [The Display Control Register](ch1/the_display_control_register.md)
* [Video Memory Intro](ch1/video_memory_intro.md)
* [hello2](ch1/hello2.md)
* [Ch 2: User Input](ch2/index.md)
* [The Key Input Register](ch2/the_key_input_register.md)
* [The VCount Register](ch2/the_vcount_register.md)
* [light_cycle](ch2/light_cycle.md)

8
book/src/ch0/index.md
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@ -4,6 +4,10 @@ Before you can build a GBA game you'll have to follow some special steps to
setup the development environment. Perhaps unfortunately, there's enough detail
here to warrant a mini-chapter all on its own.
Before we begin I'd like to give a special thanks to **Ketsuban**, who is the
wizard that arranged for all of this to be able to happen and laid out the
details of the plan to the rest of the world.
## Per System Setup
Obviously you need your computer to have a working rust installation. However,
@ -51,6 +55,10 @@ repo](https://github.com/rust-console/gba).
## Compiling
The next steps only work once you've got some source code to build. If you need
a quick test, copy the `hello1.rs` file from our examples directory in the
repository.
Once you've got something to build, you perform the following steps:
* `arm-none-eabi-as crt0.s -o crt0.o`

21
book/src/ch1/hello1.md
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@ -96,6 +96,11 @@ If you think about it for a moment you'll probably realize that, those inputs
and outputs are totally useless to us on a GBA. There's no OS on the GBA to call
our program, and there's no place for our program to "return to" when it's done.
Side note: if you want to learn more about stuff "before main gets called" you
can watch a great [CppCon talk](https://www.youtube.com/watch?v=dOfucXtyEsU) by
Matt Godbolt (yes, that Godbolt) where he delves into quite a bit of it. The
talk doesn't really apply to the GBA, but it's pretty good.
```rust
unsafe {
```
@ -163,8 +168,9 @@ anyway. Sometimes that's right, but sometimes it's wrong.
Marking a read or write as _volatile_ tells the compiler that it really must do
that action, and in the exact order that we wrote it out. It says that there
might even be special hardware side effects going on that the compiler isn't
aware of. In this case, the Display Control write sets a video mode, and the
Video RAM writes set pixels that will show up on the screen.
aware of. In this case, the write to the display control register sets a video
mode, and the writes to the Video RAM set pixels that will show up on the
screen.
Similar to "atomic" operations you might have heard about, all volatile
operations are enforced to happen in the exact order that you specify them, but
@ -176,15 +182,12 @@ a += b;
d.volatile_write(7);
```
might end up changing `a` either before or after the change to `c`, but the
write to `d` will _always_ happen after the write to `c`.
might end up changing `a` either before or after the change to `c` (since the
value of `a` doesn't affect the write to `c`), but the write to `d` will
_always_ happen after the write to `c` even though the compiler doesn't see any
direct data dependency there.
If you ever use volatile stuff on other platforms it's important to note that
volatile doesn't make things thread-safe, you still need atomic for that.
However, the GBA doesn't have threads, so we don't have to worry about thread
safety concerns.
Accordingly, our first bit of code for our library will be a _newtype_ over a
normal `*mut T` so that it has volatile reads and writes as the default. We'll
cover the details later on when we try writing a `hello2` program, once we know
more of what's going on.

2
book/src/ch1/the_display_control_register.md
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@ -73,7 +73,7 @@ Bit 6 lets you adjust if the GBA should treat Object Character VRAM as being 2d
around, so we'll be sure to have some extra diagrams in the chapter that deals
with it.
Bit 7 forces the screen to stay in vblank as long as it's set. This allows the
Bit 7 forces the screen to stay in VBlank as long as it's set. This allows the
fastest use of the VRAM, Palette, and Object Attribute Memory. Obviously if you
leave this on for too long the player will notice a blank screen, but it might
be okay to use for a moment or two every once in a while.

8
book/src/ch1/video_memory_intro.md
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@ -2,10 +2,10 @@
The GBA's Video RAM is 96k stretching from `0x0600_0000` to `0x0601_7FFF`.
The Video RAM can only be accessed totally freely during a Vertical Blank
(aka "vblank"). At other times, if the CPU tries to touch the same part of video
memory as the display controller is accessing then the CPU gets bumped by a
cycle to avoid a clash.
The Video RAM can only be accessed totally freely during a Vertical Blank (aka
"VBlank", though sometimes I forget and don't capitalize it properly). At other
times, if the CPU tries to touch the same part of video memory as the display
controller is accessing then the CPU gets bumped by a cycle to avoid a clash.
Annoyingly, VRAM can only be properly written to in 16 and 32 bit segments (same
with PALRAM and OAM). If you try to write just an 8 bit segment, then both parts

22
book/src/ch2/index.md Normal file
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@ -0,0 +1,22 @@
# Ch 2: User Input
It's all well and good to draw three pixels, but they don't do anything yet. We
want them to do something, and for that we need to get some input from the user.
The GBA, as I'm sure you know, has an arrow pad, A and B, L and R, Start and
Select. That's a little more than the NES/GB/CGB had, and a little less than the
SNES had. As you can guess, we get key state info from an IO register.
Also, we will need a way to keep the program from running "too fast". On a
modern computer or console you do this with vsync info from the GPU and Monitor,
and on the GBA we'll be using vsync info from an IO register that tracks what
the display hardware is doing.
As a way to apply our knowledge We'll make a simple "light cycle" game where
your dot leaves a trail behind them and you die if you go off the screen or if
you touch your own trail. We just make a copy of `hello2.rs` named
`light_cycle.rs` and then fill it in as we go through the chapter. Normally you
might not place the entire program into a single source file, particularly as it
grows over time, but since these are small examples it's much better to have
them be completely self contained than it is to have them be "properly
organized" for the long term.

119
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@ -0,0 +1,119 @@
# light_cycle
Now let's make a game of "light_cycle" with our new knowledge.
## Gameplay
`light_cycle` is pretty simple, and very obvious if you've ever seen Tron. The
player moves around the screen with a trail left behind them. They die if they
go off the screen or if they touch their own trail.
## Operations
We need some better drawing operations this time around.
```rust
pub unsafe fn mode3_clear_screen(color: u16) {
let color = color as u32;
let bulk_color = color << 16 | color;
let mut ptr = VRAM as *mut u32;
for _ in 0..SCREEN_HEIGHT {
for _ in 0..(SCREEN_WIDTH / 2) {
ptr.write_volatile(bulk_color);
ptr = ptr.offset(1);
}
}
}
pub unsafe fn mode3_draw_pixel(col: isize, row: isize, color: u16) {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
}
pub unsafe fn mode3_read_pixel(col: isize, row: isize) -> u16 {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).read_volatile()
}
```
The draw pixel and read pixel are both pretty obvious. What's new is the clear
screen operation. It changes the `u16` color into a `u32` and then packs the
value in twice. Then we write out `u32` values the whole way through screen
memory. This means we have to do less write operations overall, and so the
screen clear is twice as fast.
Now we just have to fill in the main function:
```rust
#[start]
fn main(_argc: isize, _argv: *const *const u8) -> isize {
unsafe {
DISPCNT.write_volatile(MODE3 | BG2);
}
let mut px = SCREEN_WIDTH / 2;
let mut py = SCREEN_HEIGHT / 2;
let mut color = rgb16(31, 0, 0);
loop {
// read the input for this frame
let this_frame_keys = read_key_input();
// adjust game state and wait for vblank
px += 2 * this_frame_keys.column_direction() as isize;
py += 2 * this_frame_keys.row_direction() as isize;
wait_until_vblank();
// draw the new game and wait until the next frame starts.
unsafe {
if px < 0 || py < 0 || px == SCREEN_WIDTH || py == SCREEN_HEIGHT {
// out of bounds, reset the screen and position.
mode3_clear_screen(0);
color = color.rotate_left(5);
px = SCREEN_WIDTH / 2;
py = SCREEN_HEIGHT / 2;
} else {
let color_here = mode3_read_pixel(px, py);
if color_here != 0 {
// crashed into our own line, reset the screen
mode3_clear_screen(0);
color = color.rotate_left(5);
} else {
// draw the new part of the line
mode3_draw_pixel(px, py, color);
mode3_draw_pixel(px, py + 1, color);
mode3_draw_pixel(px + 1, py, color);
mode3_draw_pixel(px + 1, py + 1, color);
}
}
}
wait_until_vdraw();
}
}
```
Oh that's a lot more than before!
First we set Mode 3 and Background 2, we know about that.
Then we're going to store the player's x and y, along with a color value for
their light cycle. Then we enter the core loop.
We read the keys for input, and then do as much as we can without touching video
memory. Since we're using video memory as the place to store the player's light
trail, we can't do much, we just update their position and wait for VBlank to
start. The player will be a 2x2 square, so the arrows will move you 2 pixels per
frame.
Once we're in VBlank we check to see what kind of drawing we're doing. If the
player has gone out of bounds, we clear the screen, rotate their color, and then
reset their position. Why rotate the color? Just because it's fun to have
different colors.
Next, if the player is in bounds we read the video memory for their position. If
it's not black that means we've been here before and the player has crashed into
their own line. In this case, we reset the game without moving them to a new
location.
Finally, if the player is in bounds and they haven't crashed, we write their color into memory at this position.
Regardless of how it worked out, we hold here until vdraw starts before going to
the next loop.

211
book/src/ch2/the_key_input_register.md Normal file
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@ -0,0 +1,211 @@
# The Key Input Register
The Key Input Register is our next IO register. Its shorthand name is
[KEYINPUT](http://problemkaputt.de/gbatek.htm#gbakeypadinput) and it's a `u16`
at `0x4000130`. The entire register is obviously read only, you can't tell the
GBA what buttons are pressed.
Each button is exactly one bit:
| Bit | Button |
|:---:|:------:|
| 0 | A |
| 1 | B |
| 2 | Select |
| 3 | Start |
| 4 | Right |
| 5 | Left |
| 6 | Up |
| 7 | Down |
| 8 | R |
| 9 | L |
The higher bits above are not used at all.
Similar to other old hardware devices, the convention here is that a button's
bit is **clear when pressed, active when released**. In other words, when the
user is not touching the device at all the KEYINPUT value will read
`0b0000_0011_1111_1111`. There's similar values for when the user is pressing as
many buttons as possible, but since the left/right and up/down keys are on an
arrow pad the value can never be 0 since you can't ever press every single key
at once.
When dealing with key input, the register always shows the exact key values at
any moment you read it. Obviously that's what it should do, but what it means to
you as a programmer is that you should usually gather input once at the top of a
game frame and then use that single input poll as the input values across the
whole game frame.
Of course, you might want to know if a user's key state changed from frame to
frame. That's fairly easy too: We just store the last frame keys as well as the
current frame keys (it's only a `u16`) and then we can xor the two values.
Anything that shows up in the xor result is a key that changed. If it's changed
and it's now down, that means it was pushed this frame. If it's changed and it's
now up, that means it was released this frame.
The other major thing you might frequently want is to know "which way" the arrow
pad is pointing: Up/Down/None and Left/Right/None. Sounds like an enum to me.
Except that often time we'll have situations where the direction just needs to
be multiplied by a speed and applied as a delta to a position. We want to
support that as well as we can too.
## Key Input Code
Let's get down to some code. First we want to make a way to read the address as
a `u16` and then wrap that in our newtype which will implement methods for
reading and writing the key bits.
```rust
pub const KEYINPUT: *mut u16 = 0x400_0130 as *mut u16;
/// A newtype over the key input state of the GBA.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(transparent)]
pub struct KeyInputSetting(u16);
pub fn read_key_input() -> KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile()) }
}
```
Now we want a way to check if a key is _being pressed_, since that's normally
how we think of things as a game designer and even as a player. That is, usually
you'd say "if you press A, then X happens" instead of "if you don't press A,
then X does not happen".
Normally we'd pick a constant for the bit we want, `&` it with our value, and
then check for `val != 0`. Since the bit we're looking for is `0` in the "true"
state we still pick the same constant and we still do the `&`, but we test with
`== 0`. Practically the same, right? Well, since I'm asking a rhetorical
question like that you can probably already guess that it's not the same. I was
shocked to learn this too.
All we have to do is ask our good friend
[Godbolt](https://rust.godbolt.org/z/d-8oCe) what's gonna happen when the code
compiles. The link there has the page set for the `stable` 1.30 compiler just so
that the link results stay consistent if you read this book in a year or
something. Also, we've set the target to `thumbv6m-none-eabi`, which is a
slightly later version of ARM than the actual GBA, but it's close enough for
just checking. Of course, in a full program small functions like these will
probably get inlined into the calling code and disappear entirely as they're
folded and refolded by the compiler, but we can just check.
It turns out that the `!=0` test is 4 instructions and the `==0` test is 6
instructions. Since we want to get savings where we can, and we'll probably
check the keys of an input often enough, we'll just always use a `!=0` test and
then adjust how we initially read the register to compensate.
```rust
pub fn read_key_input() -> KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile() ^ 0b1111_1111_1111_1111) }
}
```
Now we add a method for seeing if a key is pressed. In the full library there's
a more advanced version of this that's built up via macro, but for this example
we'll just name a bunch of `const` values and then have a method that takes a
value and says if that bit is on.
```rust
pub const KEY_A: u16 = 1 << 0;
pub const KEY_B: u16 = 1 << 1;
pub const KEY_SELECT: u16 = 1 << 2;
pub const KEY_START: u16 = 1 << 3;
pub const KEY_RIGHT: u16 = 1 << 4;
pub const KEY_LEFT: u16 = 1 << 5;
pub const KEY_UP: u16 = 1 << 6;
pub const KEY_DOWN: u16 = 1 << 7;
pub const KEY_R: u16 = 1 << 8;
pub const KEY_L: u16 = 1 << 9;
impl KeyInputSetting {
pub fn contains(&self, key: u16) -> bool {
(self.0 & key) != 0
}
}
```
Because each key is a unique bit you can even check for more than one key at
once by just adding two key values together.
```rust
let input_contains_a_and_l = input.contains(KEY_A + KEY_L);
```
And we wanted to save the state of an old frame and compare it to the current
frame to see what was different:
```rust
pub fn difference(&self, other: KeyInputSetting) -> KeyInputSetting {
KeyInputSetting(self.0 ^ other.0)
}
```
Anything that's "in" the difference output is a key that _changed_, and then if
the key reads as pressed this frame that means it was just pressed. The exact
mechanics of all the ways you might care to do something based on new key
presses is obviously quite varied, but it might be something like this:
```rust
let this_frame_diff = this_frame_input.difference(last_frame_input);
if this_frame_diff.contains(KEY_B) && this_frame_input.contains(KEY_B) {
// the user just pressed B, react in some way
}
```
And for the arrow pad, we'll make an enum that easily casts into `i32`. Whenever
we're working with stuff we can try to use `i32` / `isize` as often as possible
just because it's easier on the GBA's CPU if we stick to its native number size.
Having it be an enum lets us use `match` and be sure that we've covered all our
cases.
```rust
/// A "tribool" value helps us interpret the arrow pad.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(i32)]
pub enum TriBool {
Minus = -1,
Neutral = 0,
Plus = +1,
}
```
Now, how do we determine _which way_ is plus or minus? Well... I don't know.
Really. I'm not sure what the best one is because the GBA really wants the
origin at 0,0 with higher rows going down and higher cols going right. On the
other hand, all the normal math you and I learned in school is oriented with
increasing Y being upward on the page. So, at least for this demo, we're going
to go with what the GBA wants us to do and give it a try. If we don't end up
confusing ourselves then we can stick with that. Maybe we can cover it over
somehow later on.
```rust
pub fn column_direction(&self) -> TriBool {
if self.contains(KEY_RIGHT) {
TriBool::Plus
} else if self.contains(KEY_LEFT) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
pub fn row_direction(&self) -> TriBool {
if self.contains(KEY_DOWN) {
TriBool::Plus
} else if self.contains(KEY_UP) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
```
So then in our game, every frame we can check for `column_direction` and
`row_direction` and then apply those to the player's current position to make
them move around the screen.
With that settled I think we're all done with user input for now. There's some
other things to eventually know about like key interrupts that you can set and
stuff, but we'll cover that later on because it's not necessary right now.

71
book/src/ch2/the_vcount_register.md Normal file
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@ -0,0 +1,71 @@
# The VCount Register
There's an IO register called
[VCOUNT](http://problemkaputt.de/gbatek.htm#lcdiointerruptsandstatus) that shows
you, what else, the Vertical (row) COUNT(er). It's a `u16` at address
`0x0400_0006`, and it's how we'll be doing our very poor quality vertical sync
code to start.
* **What makes it poor?** Well, we're just going to read from the vcount value as
often as possible every time we need to wait for a specific value to come up,
and then proceed once it hits the point we're looking for.
* **Why is this bad?** Because we're making the CPU do a lot of useless work,
which uses a lot more power that necessary. Even if you're not on an actual
GBA you might be running inside an emulator on a phone or other handheld. You
wanna try to save battery if all you're doing with that power use is waiting
instead of making a game actually do something.
* **Can we do better?** We can, but not yet. The better way to do things is to
use a BIOS call to put the CPU into low power mode until a VBlank interrupt
happens. However, we don't know about interrupts yet, and we don't know about
BIOS calls yet, so we'll do the basic thing for now and then upgrade later.
So the way that display hardware actually displays each frame is that it moves a
tiny pointer left to right across each pixel row one pixel at a time. When it's
within the actual screen width (240px) it's drawing out those pixels. Then it
goes _past_ the edge of the screen for 68px during a period known as the
"horizontal blank" (HBlank). Then it starts on the next row and does that loop
over again. This happens for the whole screen height (160px) and then once again
it goes past the last row for another 68px into a "vertical blank" (VBlank)
period.
* One pixel is 4 CPU cycles
* HDraw is 240 pixels, HBlank is 68 pixels (1,232 cycles per full scanline)
* VDraw is 150 scanlines, VBlank is 68 scanlines (280,896 cycles per full refresh)
Now you may remember some stuff from the display control register section where
it was mentioned that some parts of memory are best accessed during VBlank, and
also during hblank with a setting applied. These blanking periods are what was
being talked about. At other times if you attempt to access video or object
memory you (the CPU) might try touching the same memory that the display device
is trying to use, in which case you get bumped back a cycle so that the display
can finish what it's doing. Also, if you really insist on doing video memory
changes while the screen is being drawn then you might get some visual glitches.
If you can, just prepare all your changes ahead of time and then assign then all
quickly during the blank period.
So first we want a way to check the vcount value at all:
```rust
pub const VCOUNT: *mut u16 = 0x0400_0006 as *mut u16;
pub fn read_vcount() -> u16 {
unsafe { VCOUNT.read_volatile() }
}
```
Then we want two little helper functions to wait until VBlank and vdraw.
```rust
pub const SCREEN_HEIGHT: isize = 160;
pub fn wait_until_vblank() {
while read_vcount() < SCREEN_HEIGHT as u16 {}
}
pub fn wait_until_vdraw() {
while read_vcount() >= SCREEN_HEIGHT as u16 {}
}
```
And... that's it. No special types to be made this time around, it's just a
number we read out of memory.

3
book/src/introduction.md
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@ -13,3 +13,6 @@ If you want to read more about developing on the GBA there are some other good r
* [GBATEK](http://problemkaputt.de/gbatek.htm), a homebrew tech manual for
GBA/NDS/DSi. We will regularly link to parts of it when talking about various
bits of the GBA.
* [CowBite](https://www.cs.rit.edu/~tjh8300/CowBite/CowBiteSpec.htm) is another
tech specification that's more GBA specific. It's sometimes got more ASCII
art diagrams and example C struct layouts than GBATEK does.

8
docs/ch0/index.html
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@ -72,7 +72,7 @@
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch0/index.html" class="active"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li></ol>
<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch0/index.html" class="active"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="../ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="../ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="../ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="../ch2/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
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@ -140,6 +140,9 @@
<p>Before you can build a GBA game you'll have to follow some special steps to
setup the development environment. Perhaps unfortunately, there's enough detail
here to warrant a mini-chapter all on its own.</p>
<p>Before we begin I'd like to give a special thanks to <strong>Ketsuban</strong>, who is the
wizard that arranged for all of this to be able to happen and laid out the
details of the plan to the rest of the world.</p>
<a class="header" href="#per-system-setup" id="per-system-setup"><h2>Per System Setup</h2></a>
<p>Obviously you need your computer to have a working rust installation. However,
you'll also need to ensure that you're using a nightly toolchain. You can run
@ -182,6 +185,9 @@ later on this is where you can put it. You also need to build it into a
that the GBA has about our program.</li>
</ul>
<a class="header" href="#compiling" id="compiling"><h2>Compiling</h2></a>
<p>The next steps only work once you've got some source code to build. If you need
a quick test, copy the <code>hello1.rs</code> file from our examples directory in the
repository.</p>
<p>Once you've got something to build, you perform the following steps:</p>
<ul>
<li>

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<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch1/hello1.html" class="active"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="../ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="../ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="../ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="../ch2/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
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@ -220,6 +220,10 @@ how <code>#[start]</code> is specified to work in Rust.</p>
<p>If you think about it for a moment you'll probably realize that, those inputs
and outputs are totally useless to us on a GBA. There's no OS on the GBA to call
our program, and there's no place for our program to &quot;return to&quot; when it's done.</p>
<p>Side note: if you want to learn more about stuff &quot;before main gets called&quot; you
can watch a great <a href="https://www.youtube.com/watch?v=dOfucXtyEsU">CppCon talk</a> by
Matt Godbolt (yes, that Godbolt) where he delves into quite a bit of it. The
talk doesn't really apply to the GBA, but it's pretty good.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
@ -280,8 +284,9 @@ anyway. Sometimes that's right, but sometimes it's wrong.</p>
<p>Marking a read or write as <em>volatile</em> tells the compiler that it really must do
that action, and in the exact order that we wrote it out. It says that there
might even be special hardware side effects going on that the compiler isn't
aware of. In this case, the Display Control write sets a video mode, and the
Video RAM writes set pixels that will show up on the screen.</p>
aware of. In this case, the write to the display control register sets a video
mode, and the writes to the Video RAM set pixels that will show up on the
screen.</p>
<p>Similar to &quot;atomic&quot; operations you might have heard about, all volatile
operations are enforced to happen in the exact order that you specify them, but
only relative to other volatile operations. So something like</p>
@ -292,16 +297,14 @@ c.volatile_write(5);
a += b;
d.volatile_write(7);
#}</code></pre></pre>
<p>might end up changing <code>a</code> either before or after the change to <code>c</code>, but the
write to <code>d</code> will <em>always</em> happen after the write to <code>c</code>.</p>
<p>might end up changing <code>a</code> either before or after the change to <code>c</code> (since the
value of <code>a</code> doesn't affect the write to <code>c</code>), but the write to <code>d</code> will
<em>always</em> happen after the write to <code>c</code> even though the compiler doesn't see any
direct data dependency there.</p>
<p>If you ever use volatile stuff on other platforms it's important to note that
volatile doesn't make things thread-safe, you still need atomic for that.
However, the GBA doesn't have threads, so we don't have to worry about thread
safety concerns.</p>
<p>Accordingly, our first bit of code for our library will be a <em>newtype</em> over a
normal <code>*mut T</code> so that it has volatile reads and writes as the default. We'll
cover the details later on when we try writing a <code>hello2</code> program, once we know
more of what's going on.</p>
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@ -250,6 +250,10 @@ purposes it's often best to do it yourself at least once.</p>
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@ -264,6 +268,10 @@ purposes it's often best to do it yourself at least once.</p>
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@ -193,7 +193,7 @@ the maximum sprites per scanline, so it's not default.</p>
(off) or 1d (on). This particular control can be kinda tricky to wrap your head
around, so we'll be sure to have some extra diagrams in the chapter that deals
with it.</p>
<p>Bit 7 forces the screen to stay in vblank as long as it's set. This allows the
<p>Bit 7 forces the screen to stay in VBlank as long as it's set. This allows the
fastest use of the VRAM, Palette, and Object Attribute Memory. Obviously if you
leave this on for too long the player will notice a blank screen, but it might
be okay to use for a moment or two every once in a while.</p>

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<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="../ch1/video_memory_intro.html" class="active"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="../ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="../ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="../ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="../ch2/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
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@ -138,10 +138,10 @@
<main>
<a class="header" href="#video-memory-intro" id="video-memory-intro"><h1>Video Memory Intro</h1></a>
<p>The GBA's Video RAM is 96k stretching from <code>0x0600_0000</code> to <code>0x0601_7FFF</code>.</p>
<p>The Video RAM can only be accessed totally freely during a Vertical Blank
(aka &quot;vblank&quot;). At other times, if the CPU tries to touch the same part of video
memory as the display controller is accessing then the CPU gets bumped by a
cycle to avoid a clash.</p>
<p>The Video RAM can only be accessed totally freely during a Vertical Blank (aka
&quot;VBlank&quot;, though sometimes I forget and don't capitalize it properly). At other
times, if the CPU tries to touch the same part of video memory as the display
controller is accessing then the CPU gets bumped by a cycle to avoid a clash.</p>
<p>Annoyingly, VRAM can only be properly written to in 16 and 32 bit segments (same
with PALRAM and OAM). If you try to write just an 8 bit segment, then both parts
of the 16 bit segment get the same value written to them. In other words, if you

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<a class="header" href="#ch-2-user-input" id="ch-2-user-input"><h1>Ch 2: User Input</h1></a>
<p>It's all well and good to draw three pixels, but they don't do anything yet. We
want them to do something, and for that we need to get some input from the user.</p>
<p>The GBA, as I'm sure you know, has an arrow pad, A and B, L and R, Start and
Select. That's a little more than the NES/GB/CGB had, and a little less than the
SNES had. As you can guess, we get key state info from an IO register.</p>
<p>Also, we will need a way to keep the program from running &quot;too fast&quot;. On a
modern computer or console you do this with vsync info from the GPU and Monitor,
and on the GBA we'll be using vsync info from an IO register that tracks what
the display hardware is doing.</p>
<p>As a way to apply our knowledge We'll make a simple &quot;light cycle&quot; game where
your dot leaves a trail behind them and you die if you go off the screen or if
you touch your own trail. We just make a copy of <code>hello2.rs</code> named
<code>light_cycle.rs</code> and then fill it in as we go through the chapter. Normally you
might not place the entire program into a single source file, particularly as it
grows over time, but since these are small examples it's much better to have
them be completely self contained than it is to have them be &quot;properly
organized&quot; for the long term.</p>
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<a class="header" href="#light_cycle" id="light_cycle"><h1>light_cycle</h1></a>
<p>Now let's make a game of &quot;light_cycle&quot; with our new knowledge.</p>
<a class="header" href="#gameplay" id="gameplay"><h2>Gameplay</h2></a>
<p><code>light_cycle</code> is pretty simple, and very obvious if you've ever seen Tron. The
player moves around the screen with a trail left behind them. They die if they
go off the screen or if they touch their own trail.</p>
<a class="header" href="#operations" id="operations"><h2>Operations</h2></a>
<p>We need some better drawing operations this time around.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub unsafe fn mode3_clear_screen(color: u16) {
let color = color as u32;
let bulk_color = color &lt;&lt; 16 | color;
let mut ptr = VRAM as *mut u32;
for _ in 0..SCREEN_HEIGHT {
for _ in 0..(SCREEN_WIDTH / 2) {
ptr.write_volatile(bulk_color);
ptr = ptr.offset(1);
}
}
}
pub unsafe fn mode3_draw_pixel(col: isize, row: isize, color: u16) {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
}
pub unsafe fn mode3_read_pixel(col: isize, row: isize) -&gt; u16 {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).read_volatile()
}
#}</code></pre></pre>
<p>The draw pixel and read pixel are both pretty obvious. What's new is the clear
screen operation. It changes the <code>u16</code> color into a <code>u32</code> and then packs the
value in twice. Then we write out <code>u32</code> values the whole way through screen
memory. This means we have to do less write operations overall, and so the
screen clear is twice as fast.</p>
<p>Now we just have to fill in the main function:</p>
<pre><pre class="playpen"><code class="language-rust">#[start]
fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
unsafe {
DISPCNT.write_volatile(MODE3 | BG2);
}
let mut px = SCREEN_WIDTH / 2;
let mut py = SCREEN_HEIGHT / 2;
let mut color = rgb16(31, 0, 0);
loop {
// read the input for this frame
let this_frame_keys = read_key_input();
// adjust game state and wait for vblank
px += 2 * this_frame_keys.column_direction() as isize;
py += 2 * this_frame_keys.row_direction() as isize;
wait_until_vblank();
// draw the new game and wait until the next frame starts.
unsafe {
if px &lt; 0 || py &lt; 0 || px == SCREEN_WIDTH || py == SCREEN_HEIGHT {
// out of bounds, reset the screen and position.
mode3_clear_screen(0);
color = color.rotate_left(5);
px = SCREEN_WIDTH / 2;
py = SCREEN_HEIGHT / 2;
} else {
let color_here = mode3_read_pixel(px, py);
if color_here != 0 {
// crashed into our own line, reset the screen
mode3_clear_screen(0);
color = color.rotate_left(5);
} else {
// draw the new part of the line
mode3_draw_pixel(px, py, color);
mode3_draw_pixel(px, py + 1, color);
mode3_draw_pixel(px + 1, py, color);
mode3_draw_pixel(px + 1, py + 1, color);
}
}
}
wait_until_vdraw();
}
}
</code></pre></pre>
<p>Oh that's a lot more than before!</p>
<p>First we set Mode 3 and Background 2, we know about that.</p>
<p>Then we're going to store the player's x and y, along with a color value for
their light cycle. Then we enter the core loop.</p>
<p>We read the keys for input, and then do as much as we can without touching video
memory. Since we're using video memory as the place to store the player's light
trail, we can't do much, we just update their position and wait for VBlank to
start. The player will be a 2x2 square, so the arrows will move you 2 pixels per
frame.</p>
<p>Once we're in VBlank we check to see what kind of drawing we're doing. If the
player has gone out of bounds, we clear the screen, rotate their color, and then
reset their position. Why rotate the color? Just because it's fun to have
different colors.</p>
<p>Next, if the player is in bounds we read the video memory for their position. If
it's not black that means we've been here before and the player has crashed into
their own line. In this case, we reset the game without moving them to a new
location.</p>
<p>Finally, if the player is in bounds and they haven't crashed, we write their color into memory at this position.</p>
<p>Regardless of how it worked out, we hold here until vdraw starts before going to
the next loop.</p>
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<a class="header" href="#the-key-input-register" id="the-key-input-register"><h1>The Key Input Register</h1></a>
<p>The Key Input Register is our next IO register. Its shorthand name is
<a href="http://problemkaputt.de/gbatek.htm#gbakeypadinput">KEYINPUT</a> and it's a <code>u16</code>
at <code>0x4000130</code>. The entire register is obviously read only, you can't tell the
GBA what buttons are pressed.</p>
<p>Each button is exactly one bit:</p>
<table><thead><tr><th align="center"> Bit </th><th align="center"> Button </th></tr></thead><tbody>
<tr><td align="center"> 0 </td><td align="center"> A </td></tr>
<tr><td align="center"> 1 </td><td align="center"> B </td></tr>
<tr><td align="center"> 2 </td><td align="center"> Select </td></tr>
<tr><td align="center"> 3 </td><td align="center"> Start </td></tr>
<tr><td align="center"> 4 </td><td align="center"> Right </td></tr>
<tr><td align="center"> 5 </td><td align="center"> Left </td></tr>
<tr><td align="center"> 6 </td><td align="center"> Up </td></tr>
<tr><td align="center"> 7 </td><td align="center"> Down </td></tr>
<tr><td align="center"> 8 </td><td align="center"> R </td></tr>
<tr><td align="center"> 9 </td><td align="center"> L </td></tr>
</tbody></table>
<p>The higher bits above are not used at all.</p>
<p>Similar to other old hardware devices, the convention here is that a button's
bit is <strong>clear when pressed, active when released</strong>. In other words, when the
user is not touching the device at all the KEYINPUT value will read
<code>0b0000_0011_1111_1111</code>. There's similar values for when the user is pressing as
many buttons as possible, but since the left/right and up/down keys are on an
arrow pad the value can never be 0 since you can't ever press every single key
at once.</p>
<p>When dealing with key input, the register always shows the exact key values at
any moment you read it. Obviously that's what it should do, but what it means to
you as a programmer is that you should usually gather input once at the top of a
game frame and then use that single input poll as the input values across the
whole game frame.</p>
<p>Of course, you might want to know if a user's key state changed from frame to
frame. That's fairly easy too: We just store the last frame keys as well as the
current frame keys (it's only a <code>u16</code>) and then we can xor the two values.
Anything that shows up in the xor result is a key that changed. If it's changed
and it's now down, that means it was pushed this frame. If it's changed and it's
now up, that means it was released this frame.</p>
<p>The other major thing you might frequently want is to know &quot;which way&quot; the arrow
pad is pointing: Up/Down/None and Left/Right/None. Sounds like an enum to me.
Except that often time we'll have situations where the direction just needs to
be multiplied by a speed and applied as a delta to a position. We want to
support that as well as we can too.</p>
<a class="header" href="#key-input-code" id="key-input-code"><h2>Key Input Code</h2></a>
<p>Let's get down to some code. First we want to make a way to read the address as
a <code>u16</code> and then wrap that in our newtype which will implement methods for
reading and writing the key bits.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const KEYINPUT: *mut u16 = 0x400_0130 as *mut u16;
/// A newtype over the key input state of the GBA.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(transparent)]
pub struct KeyInputSetting(u16);
pub fn read_key_input() -&gt; KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile()) }
}
#}</code></pre></pre>
<p>Now we want a way to check if a key is <em>being pressed</em>, since that's normally
how we think of things as a game designer and even as a player. That is, usually
you'd say &quot;if you press A, then X happens&quot; instead of &quot;if you don't press A,
then X does not happen&quot;.</p>
<p>Normally we'd pick a constant for the bit we want, <code>&amp;</code> it with our value, and
then check for <code>val != 0</code>. Since the bit we're looking for is <code>0</code> in the &quot;true&quot;
state we still pick the same constant and we still do the <code>&amp;</code>, but we test with
<code>== 0</code>. Practically the same, right? Well, since I'm asking a rhetorical
question like that you can probably already guess that it's not the same. I was
shocked to learn this too.</p>
<p>All we have to do is ask our good friend
<a href="https://rust.godbolt.org/z/d-8oCe">Godbolt</a> what's gonna happen when the code
compiles. The link there has the page set for the <code>stable</code> 1.30 compiler just so
that the link results stay consistent if you read this book in a year or
something. Also, we've set the target to <code>thumbv6m-none-eabi</code>, which is a
slightly later version of ARM than the actual GBA, but it's close enough for
just checking. Of course, in a full program small functions like these will
probably get inlined into the calling code and disappear entirely as they're
folded and refolded by the compiler, but we can just check.</p>
<p>It turns out that the <code>!=0</code> test is 4 instructions and the <code>==0</code> test is 6
instructions. Since we want to get savings where we can, and we'll probably
check the keys of an input often enough, we'll just always use a <code>!=0</code> test and
then adjust how we initially read the register to compensate.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn read_key_input() -&gt; KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile() ^ 0b1111_1111_1111_1111) }
}
#}</code></pre></pre>
<p>Now we add a method for seeing if a key is pressed. In the full library there's
a more advanced version of this that's built up via macro, but for this example
we'll just name a bunch of <code>const</code> values and then have a method that takes a
value and says if that bit is on.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const KEY_A: u16 = 1 &lt;&lt; 0;
pub const KEY_B: u16 = 1 &lt;&lt; 1;
pub const KEY_SELECT: u16 = 1 &lt;&lt; 2;
pub const KEY_START: u16 = 1 &lt;&lt; 3;
pub const KEY_RIGHT: u16 = 1 &lt;&lt; 4;
pub const KEY_LEFT: u16 = 1 &lt;&lt; 5;
pub const KEY_UP: u16 = 1 &lt;&lt; 6;
pub const KEY_DOWN: u16 = 1 &lt;&lt; 7;
pub const KEY_R: u16 = 1 &lt;&lt; 8;
pub const KEY_L: u16 = 1 &lt;&lt; 9;
impl KeyInputSetting {
pub fn contains(&amp;self, key: u16) -&gt; bool {
(self.0 &amp; key) != 0
}
}
#}</code></pre></pre>
<p>Because each key is a unique bit you can even check for more than one key at
once by just adding two key values together.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let input_contains_a_and_l = input.contains(KEY_A + KEY_L);
#}</code></pre></pre>
<p>And we wanted to save the state of an old frame and compare it to the current
frame to see what was different:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn difference(&amp;self, other: KeyInputSetting) -&gt; KeyInputSetting {
KeyInputSetting(self.0 ^ other.0)
}
#}</code></pre></pre>
<p>Anything that's &quot;in&quot; the difference output is a key that <em>changed</em>, and then if
the key reads as pressed this frame that means it was just pressed. The exact
mechanics of all the ways you might care to do something based on new key
presses is obviously quite varied, but it might be something like this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let this_frame_diff = this_frame_input.difference(last_frame_input);
if this_frame_diff.contains(KEY_B) &amp;&amp; this_frame_input.contains(KEY_B) {
// the user just pressed B, react in some way
}
#}</code></pre></pre>
<p>And for the arrow pad, we'll make an enum that easily casts into <code>i32</code>. Whenever
we're working with stuff we can try to use <code>i32</code> / <code>isize</code> as often as possible
just because it's easier on the GBA's CPU if we stick to its native number size.
Having it be an enum lets us use <code>match</code> and be sure that we've covered all our
cases.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
/// A &quot;tribool&quot; value helps us interpret the arrow pad.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(i32)]
pub enum TriBool {
Minus = -1,
Neutral = 0,
Plus = +1,
}
#}</code></pre></pre>
<p>Now, how do we determine <em>which way</em> is plus or minus? Well... I don't know.
Really. I'm not sure what the best one is because the GBA really wants the
origin at 0,0 with higher rows going down and higher cols going right. On the
other hand, all the normal math you and I learned in school is oriented with
increasing Y being upward on the page. So, at least for this demo, we're going
to go with what the GBA wants us to do and give it a try. If we don't end up
confusing ourselves then we can stick with that. Maybe we can cover it over
somehow later on.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn column_direction(&amp;self) -&gt; TriBool {
if self.contains(KEY_RIGHT) {
TriBool::Plus
} else if self.contains(KEY_LEFT) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
pub fn row_direction(&amp;self) -&gt; TriBool {
if self.contains(KEY_DOWN) {
TriBool::Plus
} else if self.contains(KEY_UP) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
#}</code></pre></pre>
<p>So then in our game, every frame we can check for <code>column_direction</code> and
<code>row_direction</code> and then apply those to the player's current position to make
them move around the screen.</p>
<p>With that settled I think we're all done with user input for now. There's some
other things to eventually know about like key interrupts that you can set and
stuff, but we'll cover that later on because it's not necessary right now.</p>
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<a class="header" href="#the-vcount-register" id="the-vcount-register"><h1>The VCount Register</h1></a>
<p>There's an IO register called
<a href="http://problemkaputt.de/gbatek.htm#lcdiointerruptsandstatus">VCOUNT</a> that shows
you, what else, the Vertical (row) COUNT(er). It's a <code>u16</code> at address
<code>0x0400_0006</code>, and it's how we'll be doing our very poor quality vertical sync
code to start.</p>
<ul>
<li><strong>What makes it poor?</strong> Well, we're just going to read from the vcount value as
often as possible every time we need to wait for a specific value to come up,
and then proceed once it hits the point we're looking for.</li>
<li><strong>Why is this bad?</strong> Because we're making the CPU do a lot of useless work,
which uses a lot more power that necessary. Even if you're not on an actual
GBA you might be running inside an emulator on a phone or other handheld. You
wanna try to save battery if all you're doing with that power use is waiting
instead of making a game actually do something.</li>
<li><strong>Can we do better?</strong> We can, but not yet. The better way to do things is to
use a BIOS call to put the CPU into low power mode until a VBlank interrupt
happens. However, we don't know about interrupts yet, and we don't know about
BIOS calls yet, so we'll do the basic thing for now and then upgrade later.</li>
</ul>
<p>So the way that display hardware actually displays each frame is that it moves a
tiny pointer left to right across each pixel row one pixel at a time. When it's
within the actual screen width (240px) it's drawing out those pixels. Then it
goes <em>past</em> the edge of the screen for 68px during a period known as the
&quot;horizontal blank&quot; (HBlank). Then it starts on the next row and does that loop
over again. This happens for the whole screen height (160px) and then once again
it goes past the last row for another 68px into a &quot;vertical blank&quot; (VBlank)
period.</p>
<ul>
<li>One pixel is 4 CPU cycles</li>
<li>HDraw is 240 pixels, HBlank is 68 pixels (1,232 cycles per full scanline)</li>
<li>VDraw is 150 scanlines, VBlank is 68 scanlines (280,896 cycles per full refresh)</li>
</ul>
<p>Now you may remember some stuff from the display control register section where
it was mentioned that some parts of memory are best accessed during VBlank, and
also during hblank with a setting applied. These blanking periods are what was
being talked about. At other times if you attempt to access video or object
memory you (the CPU) might try touching the same memory that the display device
is trying to use, in which case you get bumped back a cycle so that the display
can finish what it's doing. Also, if you really insist on doing video memory
changes while the screen is being drawn then you might get some visual glitches.
If you can, just prepare all your changes ahead of time and then assign then all
quickly during the blank period.</p>
<p>So first we want a way to check the vcount value at all:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const VCOUNT: *mut u16 = 0x0400_0006 as *mut u16;
pub fn read_vcount() -&gt; u16 {
unsafe { VCOUNT.read_volatile() }
}
#}</code></pre></pre>
<p>Then we want two little helper functions to wait until VBlank and vdraw.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const SCREEN_HEIGHT: isize = 160;
pub fn wait_until_vblank() {
while read_vcount() &lt; SCREEN_HEIGHT as u16 {}
}
pub fn wait_until_vdraw() {
while read_vcount() &gt;= SCREEN_HEIGHT as u16 {}
}
#}</code></pre></pre>
<p>And... that's it. No special types to be made this time around, it's just a
number we read out of memory.</p>
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<ol class="chapter"><li><a href="introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li></ol>
<ol class="chapter"><li><a href="introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="ch2/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
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@ -147,6 +147,9 @@ for C, but it's what I based the ordering of this book's sections on.</li>
<li><a href="http://problemkaputt.de/gbatek.htm">GBATEK</a>, a homebrew tech manual for
GBA/NDS/DSi. We will regularly link to parts of it when talking about various
bits of the GBA.</li>
<li><a href="https://www.cs.rit.edu/%7Etjh8300/CowBite/CowBiteSpec.htm">CowBite</a> is another
tech specification that's more GBA specific. It's sometimes got more ASCII
art diagrams and example C struct layouts than GBATEK does.</li>
</ul>
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<ol class="chapter"><li><a href="introduction.html" class="active"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="ch2/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
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@ -147,6 +147,9 @@ for C, but it's what I based the ordering of this book's sections on.</li>
<li><a href="http://problemkaputt.de/gbatek.htm">GBATEK</a>, a homebrew tech manual for
GBA/NDS/DSi. We will regularly link to parts of it when talking about various
bits of the GBA.</li>
<li><a href="https://www.cs.rit.edu/%7Etjh8300/CowBite/CowBiteSpec.htm">CowBite</a> is another
tech specification that's more GBA specific. It's sometimes got more ASCII
art diagrams and example C struct layouts than GBATEK does.</li>
</ul>
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<ol class="chapter"><li><a href="introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li></ol>
<ol class="chapter"><li><a href="introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="ch2/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
</nav>
<div id="page-wrapper" class="page-wrapper">
@ -147,11 +147,17 @@ for C, but it's what I based the ordering of this book's sections on.</li>
<li><a href="http://problemkaputt.de/gbatek.htm">GBATEK</a>, a homebrew tech manual for
GBA/NDS/DSi. We will regularly link to parts of it when talking about various
bits of the GBA.</li>
<li><a href="https://www.cs.rit.edu/%7Etjh8300/CowBite/CowBiteSpec.htm">CowBite</a> is another
tech specification that's more GBA specific. It's sometimes got more ASCII
art diagrams and example C struct layouts than GBATEK does.</li>
</ul>
<a class="header" href="#chapter-0-development-setup" id="chapter-0-development-setup"><h1>Chapter 0: Development Setup</h1></a>
<p>Before you can build a GBA game you'll have to follow some special steps to
setup the development environment. Perhaps unfortunately, there's enough detail
here to warrant a mini-chapter all on its own.</p>
<p>Before we begin I'd like to give a special thanks to <strong>Ketsuban</strong>, who is the
wizard that arranged for all of this to be able to happen and laid out the
details of the plan to the rest of the world.</p>
<a class="header" href="#per-system-setup" id="per-system-setup"><h2>Per System Setup</h2></a>
<p>Obviously you need your computer to have a working rust installation. However,
you'll also need to ensure that you're using a nightly toolchain. You can run
@ -194,6 +200,9 @@ later on this is where you can put it. You also need to build it into a
that the GBA has about our program.</li>
</ul>
<a class="header" href="#compiling" id="compiling"><h2>Compiling</h2></a>
<p>The next steps only work once you've got some source code to build. If you need
a quick test, copy the <code>hello1.rs</code> file from our examples directory in the
repository.</p>
<p>Once you've got something to build, you perform the following steps:</p>
<ul>
<li>
@ -374,6 +383,10 @@ how <code>#[start]</code> is specified to work in Rust.</p>
<p>If you think about it for a moment you'll probably realize that, those inputs
and outputs are totally useless to us on a GBA. There's no OS on the GBA to call
our program, and there's no place for our program to &quot;return to&quot; when it's done.</p>
<p>Side note: if you want to learn more about stuff &quot;before main gets called&quot; you
can watch a great <a href="https://www.youtube.com/watch?v=dOfucXtyEsU">CppCon talk</a> by
Matt Godbolt (yes, that Godbolt) where he delves into quite a bit of it. The
talk doesn't really apply to the GBA, but it's pretty good.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
@ -434,8 +447,9 @@ anyway. Sometimes that's right, but sometimes it's wrong.</p>
<p>Marking a read or write as <em>volatile</em> tells the compiler that it really must do
that action, and in the exact order that we wrote it out. It says that there
might even be special hardware side effects going on that the compiler isn't
aware of. In this case, the Display Control write sets a video mode, and the
Video RAM writes set pixels that will show up on the screen.</p>
aware of. In this case, the write to the display control register sets a video
mode, and the writes to the Video RAM set pixels that will show up on the
screen.</p>
<p>Similar to &quot;atomic&quot; operations you might have heard about, all volatile
operations are enforced to happen in the exact order that you specify them, but
only relative to other volatile operations. So something like</p>
@ -446,16 +460,14 @@ c.volatile_write(5);
a += b;
d.volatile_write(7);
#}</code></pre></pre>
<p>might end up changing <code>a</code> either before or after the change to <code>c</code>, but the
write to <code>d</code> will <em>always</em> happen after the write to <code>c</code>.</p>
<p>might end up changing <code>a</code> either before or after the change to <code>c</code> (since the
value of <code>a</code> doesn't affect the write to <code>c</code>), but the write to <code>d</code> will
<em>always</em> happen after the write to <code>c</code> even though the compiler doesn't see any
direct data dependency there.</p>
<p>If you ever use volatile stuff on other platforms it's important to note that
volatile doesn't make things thread-safe, you still need atomic for that.
However, the GBA doesn't have threads, so we don't have to worry about thread
safety concerns.</p>
<p>Accordingly, our first bit of code for our library will be a <em>newtype</em> over a
normal <code>*mut T</code> so that it has volatile reads and writes as the default. We'll
cover the details later on when we try writing a <code>hello2</code> program, once we know
more of what's going on.</p>
<a class="header" href="#io-registers" id="io-registers"><h1>IO Registers</h1></a>
<p>The GBA has a large number of <strong>IO Registers</strong> (not to be confused with CPU
registers). These are special memory locations from <code>0x04000000</code> to
@ -547,7 +559,7 @@ the maximum sprites per scanline, so it's not default.</p>
(off) or 1d (on). This particular control can be kinda tricky to wrap your head
around, so we'll be sure to have some extra diagrams in the chapter that deals
with it.</p>
<p>Bit 7 forces the screen to stay in vblank as long as it's set. This allows the
<p>Bit 7 forces the screen to stay in VBlank as long as it's set. This allows the
fastest use of the VRAM, Palette, and Object Attribute Memory. Obviously if you
leave this on for too long the player will notice a blank screen, but it might
be okay to use for a moment or two every once in a while.</p>
@ -579,10 +591,10 @@ nothing else special.</p>
newtyping as we attempt a <code>hello2</code> program.</p>
<a class="header" href="#video-memory-intro" id="video-memory-intro"><h1>Video Memory Intro</h1></a>
<p>The GBA's Video RAM is 96k stretching from <code>0x0600_0000</code> to <code>0x0601_7FFF</code>.</p>
<p>The Video RAM can only be accessed totally freely during a Vertical Blank
(aka &quot;vblank&quot;). At other times, if the CPU tries to touch the same part of video
memory as the display controller is accessing then the CPU gets bumped by a
cycle to avoid a clash.</p>
<p>The Video RAM can only be accessed totally freely during a Vertical Blank (aka
&quot;VBlank&quot;, though sometimes I forget and don't capitalize it properly). At other
times, if the CPU tries to touch the same part of video memory as the display
controller is accessing then the CPU gets bumped by a cycle to avoid a clash.</p>
<p>Annoyingly, VRAM can only be properly written to in 16 and 32 bit segments (same
with PALRAM and OAM). If you try to write just an 8 bit segment, then both parts
of the 16 bit segment get the same value written to them. In other words, if you
@ -776,6 +788,393 @@ pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
<p>Of course, in the full <code>gba</code> crate that this book is a part of we have these and
other elements all labeled and sorted out for you. Still, for educational
purposes it's often best to do it yourself at least once.</p>
<a class="header" href="#ch-2-user-input" id="ch-2-user-input"><h1>Ch 2: User Input</h1></a>
<p>It's all well and good to draw three pixels, but they don't do anything yet. We
want them to do something, and for that we need to get some input from the user.</p>
<p>The GBA, as I'm sure you know, has an arrow pad, A and B, L and R, Start and
Select. That's a little more than the NES/GB/CGB had, and a little less than the
SNES had. As you can guess, we get key state info from an IO register.</p>
<p>Also, we will need a way to keep the program from running &quot;too fast&quot;. On a
modern computer or console you do this with vsync info from the GPU and Monitor,
and on the GBA we'll be using vsync info from an IO register that tracks what
the display hardware is doing.</p>
<p>As a way to apply our knowledge We'll make a simple &quot;light cycle&quot; game where
your dot leaves a trail behind them and you die if you go off the screen or if
you touch your own trail. We just make a copy of <code>hello2.rs</code> named
<code>light_cycle.rs</code> and then fill it in as we go through the chapter. Normally you
might not place the entire program into a single source file, particularly as it
grows over time, but since these are small examples it's much better to have
them be completely self contained than it is to have them be &quot;properly
organized&quot; for the long term.</p>
<a class="header" href="#the-key-input-register" id="the-key-input-register"><h1>The Key Input Register</h1></a>
<p>The Key Input Register is our next IO register. Its shorthand name is
<a href="http://problemkaputt.de/gbatek.htm#gbakeypadinput">KEYINPUT</a> and it's a <code>u16</code>
at <code>0x4000130</code>. The entire register is obviously read only, you can't tell the
GBA what buttons are pressed.</p>
<p>Each button is exactly one bit:</p>
<table><thead><tr><th align="center"> Bit </th><th align="center"> Button </th></tr></thead><tbody>
<tr><td align="center"> 0 </td><td align="center"> A </td></tr>
<tr><td align="center"> 1 </td><td align="center"> B </td></tr>
<tr><td align="center"> 2 </td><td align="center"> Select </td></tr>
<tr><td align="center"> 3 </td><td align="center"> Start </td></tr>
<tr><td align="center"> 4 </td><td align="center"> Right </td></tr>
<tr><td align="center"> 5 </td><td align="center"> Left </td></tr>
<tr><td align="center"> 6 </td><td align="center"> Up </td></tr>
<tr><td align="center"> 7 </td><td align="center"> Down </td></tr>
<tr><td align="center"> 8 </td><td align="center"> R </td></tr>
<tr><td align="center"> 9 </td><td align="center"> L </td></tr>
</tbody></table>
<p>The higher bits above are not used at all.</p>
<p>Similar to other old hardware devices, the convention here is that a button's
bit is <strong>clear when pressed, active when released</strong>. In other words, when the
user is not touching the device at all the KEYINPUT value will read
<code>0b0000_0011_1111_1111</code>. There's similar values for when the user is pressing as
many buttons as possible, but since the left/right and up/down keys are on an
arrow pad the value can never be 0 since you can't ever press every single key
at once.</p>
<p>When dealing with key input, the register always shows the exact key values at
any moment you read it. Obviously that's what it should do, but what it means to
you as a programmer is that you should usually gather input once at the top of a
game frame and then use that single input poll as the input values across the
whole game frame.</p>
<p>Of course, you might want to know if a user's key state changed from frame to
frame. That's fairly easy too: We just store the last frame keys as well as the
current frame keys (it's only a <code>u16</code>) and then we can xor the two values.
Anything that shows up in the xor result is a key that changed. If it's changed
and it's now down, that means it was pushed this frame. If it's changed and it's
now up, that means it was released this frame.</p>
<p>The other major thing you might frequently want is to know &quot;which way&quot; the arrow
pad is pointing: Up/Down/None and Left/Right/None. Sounds like an enum to me.
Except that often time we'll have situations where the direction just needs to
be multiplied by a speed and applied as a delta to a position. We want to
support that as well as we can too.</p>
<a class="header" href="#key-input-code" id="key-input-code"><h2>Key Input Code</h2></a>
<p>Let's get down to some code. First we want to make a way to read the address as
a <code>u16</code> and then wrap that in our newtype which will implement methods for
reading and writing the key bits.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const KEYINPUT: *mut u16 = 0x400_0130 as *mut u16;
/// A newtype over the key input state of the GBA.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(transparent)]
pub struct KeyInputSetting(u16);
pub fn read_key_input() -&gt; KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile()) }
}
#}</code></pre></pre>
<p>Now we want a way to check if a key is <em>being pressed</em>, since that's normally
how we think of things as a game designer and even as a player. That is, usually
you'd say &quot;if you press A, then X happens&quot; instead of &quot;if you don't press A,
then X does not happen&quot;.</p>
<p>Normally we'd pick a constant for the bit we want, <code>&amp;</code> it with our value, and
then check for <code>val != 0</code>. Since the bit we're looking for is <code>0</code> in the &quot;true&quot;
state we still pick the same constant and we still do the <code>&amp;</code>, but we test with
<code>== 0</code>. Practically the same, right? Well, since I'm asking a rhetorical
question like that you can probably already guess that it's not the same. I was
shocked to learn this too.</p>
<p>All we have to do is ask our good friend
<a href="https://rust.godbolt.org/z/d-8oCe">Godbolt</a> what's gonna happen when the code
compiles. The link there has the page set for the <code>stable</code> 1.30 compiler just so
that the link results stay consistent if you read this book in a year or
something. Also, we've set the target to <code>thumbv6m-none-eabi</code>, which is a
slightly later version of ARM than the actual GBA, but it's close enough for
just checking. Of course, in a full program small functions like these will
probably get inlined into the calling code and disappear entirely as they're
folded and refolded by the compiler, but we can just check.</p>
<p>It turns out that the <code>!=0</code> test is 4 instructions and the <code>==0</code> test is 6
instructions. Since we want to get savings where we can, and we'll probably
check the keys of an input often enough, we'll just always use a <code>!=0</code> test and
then adjust how we initially read the register to compensate.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn read_key_input() -&gt; KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile() ^ 0b1111_1111_1111_1111) }
}
#}</code></pre></pre>
<p>Now we add a method for seeing if a key is pressed. In the full library there's
a more advanced version of this that's built up via macro, but for this example
we'll just name a bunch of <code>const</code> values and then have a method that takes a
value and says if that bit is on.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const KEY_A: u16 = 1 &lt;&lt; 0;
pub const KEY_B: u16 = 1 &lt;&lt; 1;
pub const KEY_SELECT: u16 = 1 &lt;&lt; 2;
pub const KEY_START: u16 = 1 &lt;&lt; 3;
pub const KEY_RIGHT: u16 = 1 &lt;&lt; 4;
pub const KEY_LEFT: u16 = 1 &lt;&lt; 5;
pub const KEY_UP: u16 = 1 &lt;&lt; 6;
pub const KEY_DOWN: u16 = 1 &lt;&lt; 7;
pub const KEY_R: u16 = 1 &lt;&lt; 8;
pub const KEY_L: u16 = 1 &lt;&lt; 9;
impl KeyInputSetting {
pub fn contains(&amp;self, key: u16) -&gt; bool {
(self.0 &amp; key) != 0
}
}
#}</code></pre></pre>
<p>Because each key is a unique bit you can even check for more than one key at
once by just adding two key values together.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let input_contains_a_and_l = input.contains(KEY_A + KEY_L);
#}</code></pre></pre>
<p>And we wanted to save the state of an old frame and compare it to the current
frame to see what was different:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn difference(&amp;self, other: KeyInputSetting) -&gt; KeyInputSetting {
KeyInputSetting(self.0 ^ other.0)
}
#}</code></pre></pre>
<p>Anything that's &quot;in&quot; the difference output is a key that <em>changed</em>, and then if
the key reads as pressed this frame that means it was just pressed. The exact
mechanics of all the ways you might care to do something based on new key
presses is obviously quite varied, but it might be something like this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let this_frame_diff = this_frame_input.difference(last_frame_input);
if this_frame_diff.contains(KEY_B) &amp;&amp; this_frame_input.contains(KEY_B) {
// the user just pressed B, react in some way
}
#}</code></pre></pre>
<p>And for the arrow pad, we'll make an enum that easily casts into <code>i32</code>. Whenever
we're working with stuff we can try to use <code>i32</code> / <code>isize</code> as often as possible
just because it's easier on the GBA's CPU if we stick to its native number size.
Having it be an enum lets us use <code>match</code> and be sure that we've covered all our
cases.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
/// A &quot;tribool&quot; value helps us interpret the arrow pad.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(i32)]
pub enum TriBool {
Minus = -1,
Neutral = 0,
Plus = +1,
}
#}</code></pre></pre>
<p>Now, how do we determine <em>which way</em> is plus or minus? Well... I don't know.
Really. I'm not sure what the best one is because the GBA really wants the
origin at 0,0 with higher rows going down and higher cols going right. On the
other hand, all the normal math you and I learned in school is oriented with
increasing Y being upward on the page. So, at least for this demo, we're going
to go with what the GBA wants us to do and give it a try. If we don't end up
confusing ourselves then we can stick with that. Maybe we can cover it over
somehow later on.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn column_direction(&amp;self) -&gt; TriBool {
if self.contains(KEY_RIGHT) {
TriBool::Plus
} else if self.contains(KEY_LEFT) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
pub fn row_direction(&amp;self) -&gt; TriBool {
if self.contains(KEY_DOWN) {
TriBool::Plus
} else if self.contains(KEY_UP) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
#}</code></pre></pre>
<p>So then in our game, every frame we can check for <code>column_direction</code> and
<code>row_direction</code> and then apply those to the player's current position to make
them move around the screen.</p>
<p>With that settled I think we're all done with user input for now. There's some
other things to eventually know about like key interrupts that you can set and
stuff, but we'll cover that later on because it's not necessary right now.</p>
<a class="header" href="#the-vcount-register" id="the-vcount-register"><h1>The VCount Register</h1></a>
<p>There's an IO register called
<a href="http://problemkaputt.de/gbatek.htm#lcdiointerruptsandstatus">VCOUNT</a> that shows
you, what else, the Vertical (row) COUNT(er). It's a <code>u16</code> at address
<code>0x0400_0006</code>, and it's how we'll be doing our very poor quality vertical sync
code to start.</p>
<ul>
<li><strong>What makes it poor?</strong> Well, we're just going to read from the vcount value as
often as possible every time we need to wait for a specific value to come up,
and then proceed once it hits the point we're looking for.</li>
<li><strong>Why is this bad?</strong> Because we're making the CPU do a lot of useless work,
which uses a lot more power that necessary. Even if you're not on an actual
GBA you might be running inside an emulator on a phone or other handheld. You
wanna try to save battery if all you're doing with that power use is waiting
instead of making a game actually do something.</li>
<li><strong>Can we do better?</strong> We can, but not yet. The better way to do things is to
use a BIOS call to put the CPU into low power mode until a VBlank interrupt
happens. However, we don't know about interrupts yet, and we don't know about
BIOS calls yet, so we'll do the basic thing for now and then upgrade later.</li>
</ul>
<p>So the way that display hardware actually displays each frame is that it moves a
tiny pointer left to right across each pixel row one pixel at a time. When it's
within the actual screen width (240px) it's drawing out those pixels. Then it
goes <em>past</em> the edge of the screen for 68px during a period known as the
&quot;horizontal blank&quot; (HBlank). Then it starts on the next row and does that loop
over again. This happens for the whole screen height (160px) and then once again
it goes past the last row for another 68px into a &quot;vertical blank&quot; (VBlank)
period.</p>
<ul>
<li>One pixel is 4 CPU cycles</li>
<li>HDraw is 240 pixels, HBlank is 68 pixels (1,232 cycles per full scanline)</li>
<li>VDraw is 150 scanlines, VBlank is 68 scanlines (280,896 cycles per full refresh)</li>
</ul>
<p>Now you may remember some stuff from the display control register section where
it was mentioned that some parts of memory are best accessed during VBlank, and
also during hblank with a setting applied. These blanking periods are what was
being talked about. At other times if you attempt to access video or object
memory you (the CPU) might try touching the same memory that the display device
is trying to use, in which case you get bumped back a cycle so that the display
can finish what it's doing. Also, if you really insist on doing video memory
changes while the screen is being drawn then you might get some visual glitches.
If you can, just prepare all your changes ahead of time and then assign then all
quickly during the blank period.</p>
<p>So first we want a way to check the vcount value at all:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const VCOUNT: *mut u16 = 0x0400_0006 as *mut u16;
pub fn read_vcount() -&gt; u16 {
unsafe { VCOUNT.read_volatile() }
}
#}</code></pre></pre>
<p>Then we want two little helper functions to wait until VBlank and vdraw.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub const SCREEN_HEIGHT: isize = 160;
pub fn wait_until_vblank() {
while read_vcount() &lt; SCREEN_HEIGHT as u16 {}
}
pub fn wait_until_vdraw() {
while read_vcount() &gt;= SCREEN_HEIGHT as u16 {}
}
#}</code></pre></pre>
<p>And... that's it. No special types to be made this time around, it's just a
number we read out of memory.</p>
<a class="header" href="#light_cycle" id="light_cycle"><h1>light_cycle</h1></a>
<p>Now let's make a game of &quot;light_cycle&quot; with our new knowledge.</p>
<a class="header" href="#gameplay" id="gameplay"><h2>Gameplay</h2></a>
<p><code>light_cycle</code> is pretty simple, and very obvious if you've ever seen Tron. The
player moves around the screen with a trail left behind them. They die if they
go off the screen or if they touch their own trail.</p>
<a class="header" href="#operations" id="operations"><h2>Operations</h2></a>
<p>We need some better drawing operations this time around.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub unsafe fn mode3_clear_screen(color: u16) {
let color = color as u32;
let bulk_color = color &lt;&lt; 16 | color;
let mut ptr = VRAM as *mut u32;
for _ in 0..SCREEN_HEIGHT {
for _ in 0..(SCREEN_WIDTH / 2) {
ptr.write_volatile(bulk_color);
ptr = ptr.offset(1);
}
}
}
pub unsafe fn mode3_draw_pixel(col: isize, row: isize, color: u16) {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
}
pub unsafe fn mode3_read_pixel(col: isize, row: isize) -&gt; u16 {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).read_volatile()
}
#}</code></pre></pre>
<p>The draw pixel and read pixel are both pretty obvious. What's new is the clear
screen operation. It changes the <code>u16</code> color into a <code>u32</code> and then packs the
value in twice. Then we write out <code>u32</code> values the whole way through screen
memory. This means we have to do less write operations overall, and so the
screen clear is twice as fast.</p>
<p>Now we just have to fill in the main function:</p>
<pre><pre class="playpen"><code class="language-rust">#[start]
fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
unsafe {
DISPCNT.write_volatile(MODE3 | BG2);
}
let mut px = SCREEN_WIDTH / 2;
let mut py = SCREEN_HEIGHT / 2;
let mut color = rgb16(31, 0, 0);
loop {
// read the input for this frame
let this_frame_keys = read_key_input();
// adjust game state and wait for vblank
px += 2 * this_frame_keys.column_direction() as isize;
py += 2 * this_frame_keys.row_direction() as isize;
wait_until_vblank();
// draw the new game and wait until the next frame starts.
unsafe {
if px &lt; 0 || py &lt; 0 || px == SCREEN_WIDTH || py == SCREEN_HEIGHT {
// out of bounds, reset the screen and position.
mode3_clear_screen(0);
color = color.rotate_left(5);
px = SCREEN_WIDTH / 2;
py = SCREEN_HEIGHT / 2;
} else {
let color_here = mode3_read_pixel(px, py);
if color_here != 0 {
// crashed into our own line, reset the screen
mode3_clear_screen(0);
color = color.rotate_left(5);
} else {
// draw the new part of the line
mode3_draw_pixel(px, py, color);
mode3_draw_pixel(px, py + 1, color);
mode3_draw_pixel(px + 1, py, color);
mode3_draw_pixel(px + 1, py + 1, color);
}
}
}
wait_until_vdraw();
}
}
</code></pre></pre>
<p>Oh that's a lot more than before!</p>
<p>First we set Mode 3 and Background 2, we know about that.</p>
<p>Then we're going to store the player's x and y, along with a color value for
their light cycle. Then we enter the core loop.</p>
<p>We read the keys for input, and then do as much as we can without touching video
memory. Since we're using video memory as the place to store the player's light
trail, we can't do much, we just update their position and wait for VBlank to
start. The player will be a 2x2 square, so the arrows will move you 2 pixels per
frame.</p>
<p>Once we're in VBlank we check to see what kind of drawing we're doing. If the
player has gone out of bounds, we clear the screen, rotate their color, and then
reset their position. Why rotate the color? Just because it's fun to have
different colors.</p>
<p>Next, if the player is in bounds we read the video memory for their position. If
it's not black that means we've been here before and the player has crashed into
their own line. In this case, we reset the game without moving them to a new
location.</p>
<p>Finally, if the player is in bounds and they haven't crashed, we write their color into memory at this position.</p>
<p>Regardless of how it worked out, we hold here until vdraw starts before going to
the next loop.</p>
</main>

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161
examples/light_cycle.rs Normal file
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@ -0,0 +1,161 @@
#![feature(start)]
#![no_std]
#[cfg(not(test))]
#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
loop {}
}
#[start]
fn main(_argc: isize, _argv: *const *const u8) -> isize {
unsafe {
DISPCNT.write_volatile(MODE3 | BG2);
}
let mut px = SCREEN_WIDTH / 2;
let mut py = SCREEN_HEIGHT / 2;
let mut color = rgb16(31, 0, 0);
loop {
// read the input for this frame
let this_frame_keys = read_key_input();
// adjust game state and wait for vblank
px += 2 * this_frame_keys.column_direction() as isize;
py += 2 * this_frame_keys.row_direction() as isize;
wait_until_vblank();
// draw the new game and wait until the next frame starts.
unsafe {
if px < 0 || py < 0 || px == SCREEN_WIDTH || py == SCREEN_HEIGHT {
// out of bounds, reset the screen and position.
mode3_clear_screen(0);
color = color.rotate_left(5);
px = SCREEN_WIDTH / 2;
py = SCREEN_HEIGHT / 2;
} else {
let color_here = mode3_read_pixel(px, py);
if color_here != 0 {
// crashed into our own line, reset the screen
mode3_clear_screen(0);
color = color.rotate_left(5);
} else {
// draw the new part of the line
mode3_draw_pixel(px, py, color);
mode3_draw_pixel(px, py + 1, color);
mode3_draw_pixel(px + 1, py, color);
mode3_draw_pixel(px + 1, py + 1, color);
}
}
}
wait_until_vdraw();
}
}
pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
pub const MODE3: u16 = 3;
pub const BG2: u16 = 0b100_0000_0000;
pub const VRAM: usize = 0x06000000;
pub const SCREEN_WIDTH: isize = 240;
pub const SCREEN_HEIGHT: isize = 160;
pub const fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
blue << 10 | green << 5 | red
}
pub unsafe fn mode3_clear_screen(color: u16) {
let color = color as u32;
let bulk_color = color << 16 | color;
let mut ptr = VRAM as *mut u32;
for _ in 0..SCREEN_HEIGHT {
for _ in 0..(SCREEN_WIDTH / 2) {
ptr.write_volatile(bulk_color);
ptr = ptr.offset(1);
}
}
}
pub unsafe fn mode3_draw_pixel(col: isize, row: isize, color: u16) {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
}
pub unsafe fn mode3_read_pixel(col: isize, row: isize) -> u16 {
(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).read_volatile()
}
pub const KEYINPUT: *mut u16 = 0x400_0130 as *mut u16;
/// A newtype over the key input state of the GBA.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(transparent)]
pub struct KeyInputSetting(u16);
/// A "tribool" value helps us interpret the arrow pad.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(i32)]
pub enum TriBool {
Minus = -1,
Neutral = 0,
Plus = 1,
}
pub fn read_key_input() -> KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read_volatile() ^ 0b1111_1111_1111_1111) }
}
pub const KEY_A: u16 = 1 << 0;
pub const KEY_B: u16 = 1 << 1;
pub const KEY_SELECT: u16 = 1 << 2;
pub const KEY_START: u16 = 1 << 3;
pub const KEY_RIGHT: u16 = 1 << 4;
pub const KEY_LEFT: u16 = 1 << 5;
pub const KEY_UP: u16 = 1 << 6;
pub const KEY_DOWN: u16 = 1 << 7;
pub const KEY_R: u16 = 1 << 8;
pub const KEY_L: u16 = 1 << 9;
impl KeyInputSetting {
pub fn contains(&self, key: u16) -> bool {
(self.0 & key) != 0
}
pub fn difference(&self, other: KeyInputSetting) -> KeyInputSetting {
KeyInputSetting(self.0 ^ other.0)
}
pub fn column_direction(&self) -> TriBool {
if self.contains(KEY_RIGHT) {
TriBool::Plus
} else if self.contains(KEY_LEFT) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
pub fn row_direction(&self) -> TriBool {
if self.contains(KEY_DOWN) {
TriBool::Plus
} else if self.contains(KEY_UP) {
TriBool::Minus
} else {
TriBool::Neutral
}
}
}
pub const VCOUNT: *mut u16 = 0x0400_0006 as *mut u16;
pub fn read_vcount() -> u16 {
unsafe { VCOUNT.read_volatile() }
}
pub fn wait_until_vblank() {
while read_vcount() < SCREEN_HEIGHT as u16 {}
}
pub fn wait_until_vdraw() {
while read_vcount() >= SCREEN_HEIGHT as u16 {}
}

4
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@ -1,10 +1,8 @@
error_on_line_overflow = false
fn_args_density = "Compressed"
reorder_imported_names = true
merge_imports = true
reorder_imports = true
reorder_imports_in_group = true
use_try_shorthand = true
write_mode = "Overwrite"
tab_spaces = 2
max_width = 150
color = "Never"

63
src/io_registers.rs
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@ -383,6 +383,69 @@ pub const SIODATA8: VolatilePtr<u16> = VolatilePtr(0x400012A as *mut u16);
/// Key Status
pub const KEYINPUT: VolatilePtr<u16> = VolatilePtr(0x4000130 as *mut u16);
/// A newtype over the key input state of the GBA.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[repr(transparent)]
pub struct KeyInputSetting(u16);
/// A "tribool" value helps us interpret the arrow pad.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(i32)]
#[allow(missing_docs)]
pub enum TriBool {
Minus = -1,
Neutral = 0,
Plus = 1,
}
#[allow(missing_docs)]
impl KeyInputSetting {
register_bit!(A_BIT, u16, 1 << 0, a_pressed, read_write);
register_bit!(B_BIT, u16, 1 << 1, b_pressed, read_write);
register_bit!(SELECT_BIT, u16, 1 << 2, select_pressed, read_write);
register_bit!(START_BIT, u16, 1 << 3, start_pressed, read_write);
register_bit!(RIGHT_BIT, u16, 1 << 4, right_pressed, read_write);
register_bit!(LEFT_BIT, u16, 1 << 5, left_pressed, read_write);
register_bit!(UP_BIT, u16, 1 << 6, up_pressed, read_write);
register_bit!(DOWN_BIT, u16, 1 << 7, down_pressed, read_write);
register_bit!(R_BIT, u16, 1 << 8, r_pressed, read_write);
register_bit!(L_BIT, u16, 1 << 9, l_pressed, read_write);
/// Takes the difference between these keys and another set of keys.
pub fn difference(&self, other: KeyInputSetting) -> KeyInputSetting {
KeyInputSetting(self.0 ^ other.0)
}
/// Gives the arrow pad value as a tribool, with Plus being increased column
/// value (right).
pub fn column_direction(&self) -> TriBool {
if self.right_pressed() {
TriBool::Plus
} else if self.left_pressed() {
TriBool::Minus
} else {
TriBool::Neutral
}
}
/// Gives the arrow pad value as a tribool, with Plus being increased row
/// value (down).
pub fn row_direction(&self) -> TriBool {
if self.down_pressed() {
TriBool::Plus
} else if self.up_pressed() {
TriBool::Minus
} else {
TriBool::Neutral
}
}
}
/// Gets the current state of the keys
pub fn read_key_input() -> KeyInputSetting {
unsafe { KeyInputSetting(KEYINPUT.read() ^ 0b1111_1111_1111_1111) }
}
/// Key Interrupt Control
pub const KEYCNT: VolatilePtr<u16> = VolatilePtr(0x4000132 as *mut u16);