hello world chapter is complete

README.md

@@ -30,7 +30,7 @@ do this stuff.
    dots as well as other support files:
   * crt0.s
   * linker.ld
-  * thumbv4-nintendo-agb.json
+  * 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`
@@ -38,7 +38,7 @@ do this stuff.
    `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-nintendo-agb.json`
+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
@@ -47,7 +47,7 @@ do this stuff.
     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-nintendo-agb/debug/gbatest target/output.gba`
+  * `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

book/src/SUMMARY.md

@@ -8,3 +8,4 @@
   * [IO Registers](ch1/io_registers.md)
   * [The Display Control](ch1/the_display_control.md)
   * [Video Memory Intro](ch1/video_memory_intro.md)
+  * [hello2](ch1/hello2.md)

book/src/ch0/index.md

@@ -37,10 +37,12 @@ Now you'll need some particular files each time you want to start a new project.
 You can find them in the root of the [rust-console/gba
 repo](https://github.com/rust-console/gba).
 
-* `thumbv4-nintendo-agb.json` describes the overall GBA to cargo-xbuild so it
+* `thumbv4-none-agb.json` describes the overall GBA to cargo-xbuild so it knows
-  knows what to do. This is actually a somewhat made up target name since
+  what to do. This is actually a somewhat made up target name since there's no
-  there's no official target name, but this is the best name to pick based on
+  official target name. The GBA is essentially the same as a normal
-  [how target names are decided](https://wiki.osdev.org/Target_Triplet).
+  `thumbv4-none-eabi` device, but we give it the "agb" signifier so that later
+  on we'll be able to use rust's `cfg` ability to allow our code to know if it's
+  specifically targeting a GBA or some other similar device (like an NDS).
 * `crt0.s` describes some ASM startup stuff. If you have more ASM to place here
   later on this is where you can put it. You also need to build it into a
   `crt0.o` file before it can actually be used, but we'll cover that below.
@@ -57,7 +59,7 @@ Once you've got something to build, you perform the following steps:
     might as well do it every time so that you never forget to because it's a
     practically instant operation.
 
-* `cargo xbuild --target thumbv4-nintendo-agb.json`
+* `cargo xbuild --target thumbv4-none-agb.json`
   * This builds your Rust source. It accepts _most of_ the normal options, such
     as `--release`, and options, such as `--bin foo` or `--examples`, that you'd
     expect `cargo` to accept.
@@ -83,7 +85,7 @@ emulators can also do it.
 However, if you want a "real" ROM that works in all emulators and that you could
 transfer to a flash cart there's a little more to do.
 
-* `arm-none-eabi-objcopy -O binary target/thumbv4-nintendo-agb/MODE/BIN_NAME target/ROM_NAME.gba`
+* `arm-none-eabi-objcopy -O binary target/thumbv4-none-agb/MODE/BIN_NAME target/ROM_NAME.gba`
   * This will perform an [objcopy](https://linux.die.net/man/1/objcopy) on our
     program. Here I've named the program `arm-none-eabi-objcopy`, which is what
     devkitpro calls their version of `objcopy` that's specific to the GBA in the
@@ -97,7 +99,7 @@ transfer to a flash cart there's a little more to do.
     `cargo` arranges stuff in the `target/` directory, and between RLS and
     `cargo doc` and stuff it gets kinda crowded, so it goes like this:
     * Since our program was built for a non-local target, first we've got a
-      directory named for that target, `thumbv4-nintendo-agb/`
+      directory named for that target, `thumbv4-none-agb/`
     * Next, the "MODE" is either `debug/` or `release/`, depending on if we had
       the `--release` flag included. You'll probably only be packing release
       mode programs all the way into GBA roms, but it works with either mode.

book/src/ch1/hello1.md

@@ -177,3 +177,8 @@ 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.

book/src/ch1/hello2.md

@@ -0,0 +1,114 @@
+# hello2
+
+Okay so let's have a look again:
+
+`hello1`
+
+```rust
+#![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 {
+    (0x04000000 as *mut u16).write_volatile(0x0403);
+    (0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+    loop {}
+  }
+}
+```
+
+Now let's clean this up so that it's clearer what's going on.
+
+First we'll label that display control stuff:
+
+```rust
+pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
+pub const MODE3: u16 = 3;
+pub const BG2: u16 = 0b100_0000_0000;
+```
+
+Next we make some const values for the actual pixel drawing
+
+```rust
+pub const VRAM: usize = 0x06000000;
+pub const SCREEN_WIDTH: isize = 240;
+```
+
+And then we want a small helper function for putting together a color value.
+
+Happily, this one can even be declared as a const function. At the time of
+writing, we've got the "minimal const fn" support in nightly. It really is quite
+limited, but I'm happy to let rustc and LLVM pre-compute as much as they can
+when it comes to the GBA's tiny CPU.
+
+```rust
+pub const fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
+  blue << 10 | green << 5 | red
+}
+```
+
+Finally, we'll make a function for drawing a pixel in Mode 3. Even though it's
+just a one-liner, having the "important parts" be labeled as function arguments
+usually helps you think about it a lot better.
+
+```rust
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}
+```
+
+So now we've got this:
+
+`hello2`
+
+```rust
+#![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);
+    mode3_pixel(120, 80, rgb16(31, 0, 0));
+    mode3_pixel(136, 80, rgb16(0, 31, 0));
+    mode3_pixel(120, 96, rgb16(0, 0, 31));
+    loop {}
+  }
+}
+
+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 fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
+  blue << 10 | green << 5 | red
+}
+
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}
+```
+
+Exact same program that we started with, but much easier to read.
+
+Of course, in the full `gba` 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.

book/src/ch1/the_display_control.md

@@ -27,9 +27,10 @@ own demos and chapters later on, so that's all we'll say about them for now.
 Modes 3, 4, and 5 are "Bitmap" modes. These let you write individual pixels to
 locations on the screen.
 
-* **Mode 3** is full resolution (240w x 160h) RBG15 color. You might not be used to
+* **Mode 3** is full resolution (240w x 160h) RBG15 color. You might not be used
-  RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
+  to RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
-  bits for each color channel, and the highest bit is totally ignored.
+  bits for each color channel stored within a `u16` value, and the highest bit is
+  simply ignored.
 * **Mode 4** is full resolution paletted color. Instead of being a `u16` color, each
   pixel value is a `u8` palette index entry, and then the display uses the
   palette memory (which we'll talk about later) to store the actual color data.
@@ -99,3 +100,6 @@ First let's [convert that to
 binary](https://www.wolframalpha.com/input/?i=0x0403), and we get
 `0b100_0000_0011`. So, that's setting Mode 3 with background 2 enabled and
 nothing else special.
+
+However, I think we can do better than that. This is a prime target for more
+newtyping as we attempt a `hello2` program.

book/src/ch1/video_memory_intro.md

@@ -18,20 +18,96 @@ don't know to follow the special rules.
 
 ## RGB15
 
-TODO
+As I said before, RGB15 stores a color within a `u16` value using 5 bits for
+each color channel.
+
+```rust
+pub const RED:   u16 = 0b0_00000_00000_11111;
+pub const GREEN: u16 = 0b0_00000_11111_00000;
+pub const BLUE:  u16 = 0b0_11111_00000_00000;
+```
+
+In Mode 3 and Mode 5 we write direct color values into VRAM, and in Mode 4 we
+write palette index values, and then the color values go into the PALRAM.
 
 ## Mode 3
 
-TODO
+Mode 3 is pretty easy. We have a full resolution grid of rgb15 pixels. There's
+160 rows of 240 pixels each, with the base address being the top left corner. A
+particular pixel uses normal "2d indexing" math:
+
+```rust
+let row_five_col_seven = 5 + (7 * SCREEN_WIDTH);
+```
+
+To draw a pixel, we just write a value at the address for the row and col that
+we want to draw to.
 
 ## Mode 4
 
-TODO
+Mode 4 introduces page flipping. Instead of one giant page at `0x0600_0000`,
+there's Page 0 at `0x0600_0000` and then Page 1 at `0x0600_A000`. The resolution
+for each page is the same as above, but instead of writing `u16` values, the
+memory is treated as `u8` indexes into PALRAM. The PALRAM starts at
+`0x0500_0000`, and there's enough space for 256 palette entries (each a `u16`).
+
+To set the color of a palette entry we just do a normal `u16` write_volatile.
+
+```rust
+(0x0500_0000 as *mut u16).offset(target_index).write_volatile(new_color)
+```
+
+To draw a pixel we set the palette entry that we want the pixel to use. However,
+we must remember the "minimum size" write limitation that applies to VRAM. So,
+if we want to change just a single pixel at a time we must
+
+1) Read the full `u16` it's a part of.
+2) Clear the half of the `u16` we're going to replace
+3) Write the half of the `u16` we're going to replace with the new value
+4) Write that result back to the address.
+
+So, the math for finding a byte offset is the same as Mode 3 (since they're both
+a 2d grid). If the byte offset is EVEN it'll be the high bits of the `u16` at
+half the byte offset rounded down. If the offset is ODD it'll be the low bits of
+the `u16` at half the byte.
+
+Does that make sense?
+
+* If we want to write pixel (0,0) the byte offset is 0, so we change the high
+  bits of `u16` offset 0. Then we want to write to (1,0), so the byte offset is
+  1, so we change the low bits of `u16` offset 0. The pixels are next to each
+  other, and the target bytes are next to each other, good so far.
+* If we want to write to (5,6) that'd be byte `5 + 6 * 240 = 1445`, so we'd
+  target the low bits of `u16` offset `floor(1445/2) = 722`.
+
+As you can see, trying to write individual pixels in Mode 4 is mostly a bad
+time. Fret not! We don't _have_ to write individual bytes. If our data is
+arranged correctly ahead of time we can just write `u16` or `u32` values
+directly. The video hardware doesn't care, it'll get along just fine.
 
 ## Mode 5
 
-TODO
+Mode 5 is also a two page mode, but instead of compressing the size of a pixel's
+data to fit in two pages, we compress the resolution.
+
+Mode 5 is full `u16` color, but only 160w x 128h per page.
 
 ## In Conclusion...
 
-TODO
+So what got written into VRAM in `hello1`?
+
+```rust
+    (0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+```
+
+So at pixels `(120,80)`, `(136,80)`, and `(120,96)` we write three values. Once
+again we probably need to [convert them](https://www.wolframalpha.com/) into
+binary to make sense of it.
+
+* 0x001F: 0b11111
+* 0x03E0: 0b11111_00000
+* 0x7C00: 0b11111_00000_00000
+
+Ah, of course, a red pixel, a green pixel, and a blue pixel.

build.bat

@@ -3,12 +3,12 @@
 arm-none-eabi-as crt0.s -o crt0.o
 
 @rem Build all examples, both debug and release
-cargo xbuild --examples --target thumbv4-nintendo-agb.json
+cargo xbuild --examples --target thumbv4-none-agb.json
-cargo xbuild --examples --target thumbv4-nintendo-agb.json --release
+cargo xbuild --examples --target thumbv4-none-agb.json --release
 
 @echo Packing examples into ROM files...
 @for %%I in (.\examples\*.*) do @(
   echo %%~nI
-  arm-none-eabi-objcopy -O binary target/thumbv4-nintendo-agb/release/examples/%%~nI target/example-%%~nI.gba >nul
+  arm-none-eabi-objcopy -O binary target/thumbv4-none-agb/release/examples/%%~nI target/example-%%~nI.gba >nul
   gbafix target/example-%%~nI.gba >nul
 )

docs/ch0/index.html

@@ -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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
         </nav>
 
         <div id="page-wrapper" class="page-wrapper">
@@ -169,8 +169,12 @@ cargo will figure it all out for you.</p>
 You can find them in the root of the <a href="https://github.com/rust-console/gba">rust-console/gba
 repo</a>.</p>
 <ul>
-<li><code>thumbv4-nintendo-agb.json</code> describes the overall GBA to cargo-xbuild so it knows
+<li><code>thumbv4-none-agb.json</code> describes the overall GBA to cargo-xbuild so it knows
-what to do.</li>
+what to do. This is actually a somewhat made up target name since there's no
+official target name. The GBA is essentially the same as a normal
+<code>thumbv4-none-eabi</code> device, but we give it the &quot;agb&quot; signifier so that later
+on we'll be able to use rust's <code>cfg</code> ability to allow our code to know if it's
+specifically targeting a GBA or some other similar device (like an NDS).</li>
 <li><code>crt0.s</code> describes some ASM startup stuff. If you have more ASM to place here
 later on this is where you can put it. You also need to build it into a
 <code>crt0.o</code> file before it can actually be used, but we'll cover that below.</li>
@@ -190,7 +194,7 @@ practically instant operation.</li>
 </ul>
 </li>
 <li>
-<p><code>cargo xbuild --target thumbv4-nintendo-agb.json</code></p>
+<p><code>cargo xbuild --target thumbv4-none-agb.json</code></p>
 <ul>
 <li>This builds your Rust source. It accepts <em>most of</em> the normal options, such
 as <code>--release</code>, and options, such as <code>--bin foo</code> or <code>--examples</code>, that you'd
@@ -219,7 +223,7 @@ emulators can also do it.</p>
 transfer to a flash cart there's a little more to do.</p>
 <ul>
 <li>
-<p><code>arm-none-eabi-objcopy -O binary target/thumbv4-nintendo-agb/MODE/BIN_NAME target/ROM_NAME.gba</code></p>
+<p><code>arm-none-eabi-objcopy -O binary target/thumbv4-none-agb/MODE/BIN_NAME target/ROM_NAME.gba</code></p>
 <ul>
 <li>This will perform an <a href="https://linux.die.net/man/1/objcopy">objcopy</a> on our
 program. Here I've named the program <code>arm-none-eabi-objcopy</code>, which is what
@@ -235,7 +239,7 @@ bare memory dump of the program.</li>
 <code>cargo doc</code> and stuff it gets kinda crowded, so it goes like this:
 <ul>
 <li>Since our program was built for a non-local target, first we've got a
-directory named for that target, <code>thumbv4-nintendo-agb/</code></li>
+directory named for that target, <code>thumbv4-none-agb/</code></li>
 <li>Next, the &quot;MODE&quot; is either <code>debug/</code> or <code>release/</code>, depending on if we had
 the <code>--release</code> flag included. You'll probably only be packing release
 mode programs all the way into GBA roms, but it works with either mode.</li>

docs/ch1/hello1.html

@@ -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"><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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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" 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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
         </nav>
 
         <div id="page-wrapper" class="page-wrapper">
@@ -293,6 +293,10 @@ write to <code>d</code> will <em>always</em> happen after the write to <code>c</
 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>
 
                     </main>
 

docs/ch1/hello2.html

@@ -0,0 +1,289 @@
+<!DOCTYPE HTML>
+<html lang="en" class="sidebar-visible no-js">
+    <head>
+        <!-- Book generated using mdBook -->
+        <meta charset="UTF-8">
+        <title>hello2 - Rust GBA Tutorials</title>
+        <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
+        <meta name="description" content="">
+        <meta name="viewport" content="width=device-width, initial-scale=1">
+        <meta name="theme-color" content="#ffffff" />
+
+        <link rel="shortcut icon" href="../favicon.png">
+        <link rel="stylesheet" href="../css/variables.css">
+        <link rel="stylesheet" href="../css/general.css">
+        <link rel="stylesheet" href="../css/chrome.css">
+        <link rel="stylesheet" href="../css/print.css" media="print">
+
+        <!-- Fonts -->
+        <link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
+        <link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css">
+        <link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css">
+
+        <!-- Highlight.js Stylesheets -->
+        <link rel="stylesheet" href="../highlight.css">
+        <link rel="stylesheet" href="../tomorrow-night.css">
+        <link rel="stylesheet" href="../ayu-highlight.css">
+
+        <!-- Custom theme stylesheets -->
+        
+
+        
+    </head>
+    <body class="light">
+        <!-- Provide site root to javascript -->
+        <script type="text/javascript">var path_to_root = "../";</script>
+
+        <!-- Work around some values being stored in localStorage wrapped in quotes -->
+        <script type="text/javascript">
+            try {
+                var theme = localStorage.getItem('mdbook-theme');
+                var sidebar = localStorage.getItem('mdbook-sidebar');
+
+                if (theme.startsWith('"') && theme.endsWith('"')) {
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+                        <a class="header" href="#hello2" id="hello2"><h1>hello2</h1></a>
+<p>Okay so let's have a look again:</p>
+<p><code>hello1</code></p>
+<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
+#![no_std]
+
+#[cfg(not(test))]
+#[panic_handler]
+fn panic(_info: &amp;core::panic::PanicInfo) -&gt; ! {
+  loop {}
+}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
+  unsafe {
+    (0x04000000 as *mut u16).write_volatile(0x0403);
+    (0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+    loop {}
+  }
+}
+</code></pre></pre>
+<p>Now let's clean this up so that it's clearer what's going on.</p>
+<p>First we'll label that display control stuff:</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
+pub const MODE3: u16 = 3;
+pub const BG2: u16 = 0b100_0000_0000;
+#}</code></pre></pre>
+<p>Next we make some const values for the actual pixel drawing</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const VRAM: usize = 0x06000000;
+pub const SCREEN_WIDTH: isize = 240;
+#}</code></pre></pre>
+<p>And then we want a small helper function for putting together a color value.</p>
+<p>Happily, this one can even be declared as a const function. At the time of
+writing, we've got the &quot;minimal const fn&quot; support in nightly. It really is quite
+limited, but I'm happy to let rustc and LLVM pre-compute as much as they can
+when it comes to the GBA's tiny CPU.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const fn rgb16(red: u16, green: u16, blue: u16) -&gt; u16 {
+  blue &lt;&lt; 10 | green &lt;&lt; 5 | red
+}
+#}</code></pre></pre>
+<p>Finally, we'll make a function for drawing a pixel in Mode 3. Even though it's
+just a one-liner, having the &quot;important parts&quot; be labeled as function arguments
+usually helps you think about it a lot better.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}
+#}</code></pre></pre>
+<p>So now we've got this:</p>
+<p><code>hello2</code></p>
+<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
+#![no_std]
+
+#[cfg(not(test))]
+#[panic_handler]
+fn panic(_info: &amp;core::panic::PanicInfo) -&gt; ! {
+  loop {}
+}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
+  unsafe {
+    DISPCNT.write_volatile(MODE3 | BG2);
+    mode3_pixel(120, 80, rgb16(31, 0, 0));
+    mode3_pixel(136, 80, rgb16(0, 31, 0));
+    mode3_pixel(120, 96, rgb16(0, 0, 31));
+    loop {}
+  }
+}
+
+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 fn rgb16(red: u16, green: u16, blue: u16) -&gt; u16 {
+  blue &lt;&lt; 10 | green &lt;&lt; 5 | red
+}
+
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}
+</code></pre></pre>
+
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docs/ch1/index.html

@@ -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"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html" class="active"><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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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" class="active"><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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
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@@ -72,7 +72,7 @@
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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" class="active"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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" class="active"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
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@@ -72,7 +72,7 @@
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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.html" class="active"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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.html" class="active"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
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@@ -157,9 +157,10 @@ own demos and chapters later on, so that's all we'll say about them for now.</p>
 <p>Modes 3, 4, and 5 are &quot;Bitmap&quot; modes. These let you write individual pixels to
 locations on the screen.</p>
 <ul>
-<li><strong>Mode 3</strong> is full resolution (240w x 160h) RBG15 color. You might not be used to
+<li><strong>Mode 3</strong> is full resolution (240w x 160h) RBG15 color. You might not be used
-RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
+to RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
-bits for each color channel, and the highest bit is totally ignored.</li>
+bits for each color channel stored within a <code>u16</code> value, and the highest bit is
+simply ignored.</li>
 <li><strong>Mode 4</strong> is full resolution paletted color. Instead of being a <code>u16</code> color, each
 pixel value is a <code>u8</code> palette index entry, and then the display uses the
 palette memory (which we'll talk about later) to store the actual color data.
@@ -172,9 +173,10 @@ reduced resolution to compensate (video memory is only so big!). The screen is
 effectively only 160w x 128h in this mode.</li>
 </ul>
 <a class="header" href="#cgb-mode" id="cgb-mode"><h2>CGB Mode</h2></a>
-<p>Bit 3 is read only. It's on if you're running in CGB mode. Since we're making
+<p>Bit 3 is effectively read only. Technically it can be flipped using a BIOS call,
-GBA games you'd think that it'll never be on at all, but I guess you can change
+but when you write to the display control normally it won't write to this bit,
-it with BIOS stuff. Still, basically not an important bit.</p>
+so we'll call it effectively read only.</p>
+<p>This bit is on if the CPU is in CGB mode.</p>
 <a class="header" href="#page-flipping" id="page-flipping"><h2>Page Flipping</h2></a>
 <p>Bit 4 lets you pick which page to use. This is only relevent in video modes 4 or
 5, and is just ignored otherwise. It's very easy to remember: when the bit is 0
@@ -213,6 +215,8 @@ some nifty graphical effects.</p>
 binary</a>, and we get
 <code>0b100_0000_0011</code>. So, that's setting Mode 3 with background 2 enabled and
 nothing else special.</p>
+<p>However, I think we can do better than that. This is a prime target for more
+newtyping as we attempt a <code>hello2</code> program.</p>
 
                     </main>
 

docs/ch1/video_memory_intro.html

@@ -72,7 +72,7 @@
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         <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"><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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="../ch1/video_memory_intro.html" class="active"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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></ol>
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@@ -151,15 +151,88 @@ trying to set an individual byte, and we also have to be careful if we use
 <code>memcopy</code> or <code>memset</code> as well, because they're byte oriented by default and
 don't know to follow the special rules.</p>
 <a class="header" href="#rgb15" id="rgb15"><h2>RGB15</h2></a>
-<p>TODO</p>
+<p>As I said before, RGB15 stores a color within a <code>u16</code> value using 5 bits for
+each color channel.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const RED:   u16 = 0b0_00000_00000_11111;
+pub const GREEN: u16 = 0b0_00000_11111_00000;
+pub const BLUE:  u16 = 0b0_11111_00000_00000;
+#}</code></pre></pre>
+<p>In Mode 3 and Mode 5 we write direct color values into VRAM, and in Mode 4 we
+write palette index values, and then the color values go into the PALRAM.</p>
 <a class="header" href="#mode-3" id="mode-3"><h2>Mode 3</h2></a>
-<p>TODO</p>
+<p>Mode 3 is pretty easy. We have a full resolution grid of rgb15 pixels. There's
+160 rows of 240 pixels each, with the base address being the top left corner. A
+particular pixel uses normal &quot;2d indexing&quot; math:</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+let row_five_col_seven = 5 + (7 * SCREEN_WIDTH);
+#}</code></pre></pre>
+<p>To draw a pixel, we just write a value at the address for the row and col that
+we want to draw to.</p>
 <a class="header" href="#mode-4" id="mode-4"><h2>Mode 4</h2></a>
-<p>TODO</p>
+<p>Mode 4 introduces page flipping. Instead of one giant page at <code>0x0600_0000</code>,
+there's Page 0 at <code>0x0600_0000</code> and then Page 1 at <code>0x0600_A000</code>. The resolution
+for each page is the same as above, but instead of writing <code>u16</code> values, the
+memory is treated as <code>u8</code> indexes into PALRAM. The PALRAM starts at
+<code>0x0500_0000</code>, and there's enough space for 256 palette entries (each a <code>u16</code>).</p>
+<p>To set the color of a palette entry we just do a normal <code>u16</code> write_volatile.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+(0x0500_0000 as *mut u16).offset(target_index).write_volatile(new_color)
+#}</code></pre></pre>
+<p>To draw a pixel we set the palette entry that we want the pixel to use. However,
+we must remember the &quot;minimum size&quot; write limitation that applies to VRAM. So,
+if we want to change just a single pixel at a time we must</p>
+<ol>
+<li>Read the full <code>u16</code> it's a part of.</li>
+<li>Clear the half of the <code>u16</code> we're going to replace</li>
+<li>Write the half of the <code>u16</code> we're going to replace with the new value</li>
+<li>Write that result back to the address.</li>
+</ol>
+<p>So, the math for finding a byte offset is the same as Mode 3 (since they're both
+a 2d grid). If the byte offset is EVEN it'll be the high bits of the <code>u16</code> at
+half the byte offset rounded down. If the offset is ODD it'll be the low bits of
+the <code>u16</code> at half the byte.</p>
+<p>Does that make sense?</p>
+<ul>
+<li>If we want to write pixel (0,0) the byte offset is 0, so we change the high
+bits of <code>u16</code> offset 0. Then we want to write to (1,0), so the byte offset is
+1, so we change the low bits of <code>u16</code> offset 0. The pixels are next to each
+other, and the target bytes are next to each other, good so far.</li>
+<li>If we want to write to (5,6) that'd be byte <code>5 + 6 * 240 = 1445</code>, so we'd
+target the low bits of <code>u16</code> offset <code>floor(1445/2) = 722</code>.</li>
+</ul>
+<p>As you can see, trying to write individual pixels in Mode 4 is mostly a bad
+time. Fret not! We don't <em>have</em> to write individual bytes. If our data is
+arranged correctly ahead of time we can just write <code>u16</code> or <code>u32</code> values
+directly. The video hardware doesn't care, it'll get along just fine.</p>
 <a class="header" href="#mode-5" id="mode-5"><h2>Mode 5</h2></a>
-<p>TODO</p>
+<p>Mode 5 is also a two page mode, but instead of compressing the size of a pixel's
+data to fit in two pages, we compress the resolution.</p>
+<p>Mode 5 is full <code>u16</code> color, but only 160w x 128h per page.</p>
 <a class="header" href="#in-conclusion" id="in-conclusion"><h2>In Conclusion...</h2></a>
-<p>TODO</p>
+<p>So what got written into VRAM in <code>hello1</code>?</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+    (0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+#}</code></pre></pre>
+<p>So at pixels <code>(120,80)</code>, <code>(136,80)</code>, and <code>(120,96)</code> we write three values. Once
+again we probably need to <a href="https://www.wolframalpha.com/">convert them</a> into
+binary to make sense of it.</p>
+<ul>
+<li>0x001F: 0b11111</li>
+<li>0x03E0: 0b11111_00000</li>
+<li>0x7C00: 0b11111_00000_00000</li>
+</ul>
+<p>Ah, of course, a red pixel, a green pixel, and a blue pixel.</p>
 
                     </main>
 
@@ -172,6 +245,10 @@ don't know to follow the special rules.</p>
                         
 
                         
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@@ -186,6 +263,10 @@ don't know to follow the special rules.</p>
                 
 
                 
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@@ -72,7 +72,7 @@
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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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
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@@ -72,7 +72,7 @@
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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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li></ol></li></ol>
+            <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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
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@@ -72,7 +72,7 @@
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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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</a></li><li><a href="ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</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.html"><strong aria-hidden="true">3.3.</strong> The Display Control</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>
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@@ -181,8 +181,12 @@ cargo will figure it all out for you.</p>
 You can find them in the root of the <a href="https://github.com/rust-console/gba">rust-console/gba
 repo</a>.</p>
 <ul>
-<li><code>thumbv4-nintendo-agb.json</code> describes the overall GBA to cargo-xbuild so it knows
+<li><code>thumbv4-none-agb.json</code> describes the overall GBA to cargo-xbuild so it knows
-what to do.</li>
+what to do. This is actually a somewhat made up target name since there's no
+official target name. The GBA is essentially the same as a normal
+<code>thumbv4-none-eabi</code> device, but we give it the &quot;agb&quot; signifier so that later
+on we'll be able to use rust's <code>cfg</code> ability to allow our code to know if it's
+specifically targeting a GBA or some other similar device (like an NDS).</li>
 <li><code>crt0.s</code> describes some ASM startup stuff. If you have more ASM to place here
 later on this is where you can put it. You also need to build it into a
 <code>crt0.o</code> file before it can actually be used, but we'll cover that below.</li>
@@ -202,7 +206,7 @@ practically instant operation.</li>
 </ul>
 </li>
 <li>
-<p><code>cargo xbuild --target thumbv4-nintendo-agb.json</code></p>
+<p><code>cargo xbuild --target thumbv4-none-agb.json</code></p>
 <ul>
 <li>This builds your Rust source. It accepts <em>most of</em> the normal options, such
 as <code>--release</code>, and options, such as <code>--bin foo</code> or <code>--examples</code>, that you'd
@@ -231,7 +235,7 @@ emulators can also do it.</p>
 transfer to a flash cart there's a little more to do.</p>
 <ul>
 <li>
-<p><code>arm-none-eabi-objcopy -O binary target/thumbv4-nintendo-agb/MODE/BIN_NAME target/ROM_NAME.gba</code></p>
+<p><code>arm-none-eabi-objcopy -O binary target/thumbv4-none-agb/MODE/BIN_NAME target/ROM_NAME.gba</code></p>
 <ul>
 <li>This will perform an <a href="https://linux.die.net/man/1/objcopy">objcopy</a> on our
 program. Here I've named the program <code>arm-none-eabi-objcopy</code>, which is what
@@ -247,7 +251,7 @@ bare memory dump of the program.</li>
 <code>cargo doc</code> and stuff it gets kinda crowded, so it goes like this:
 <ul>
 <li>Since our program was built for a non-local target, first we've got a
-directory named for that target, <code>thumbv4-nintendo-agb/</code></li>
+directory named for that target, <code>thumbv4-none-agb/</code></li>
 <li>Next, the &quot;MODE&quot; is either <code>debug/</code> or <code>release/</code>, depending on if we had
 the <code>--release</code> flag included. You'll probably only be packing release
 mode programs all the way into GBA roms, but it works with either mode.</li>
@@ -443,6 +447,10 @@ write to <code>d</code> will <em>always</em> happen after the write to <code>c</
 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
@@ -498,9 +506,10 @@ own demos and chapters later on, so that's all we'll say about them for now.</p>
 <p>Modes 3, 4, and 5 are &quot;Bitmap&quot; modes. These let you write individual pixels to
 locations on the screen.</p>
 <ul>
-<li><strong>Mode 3</strong> is full resolution (240w x 160h) RBG15 color. You might not be used to
+<li><strong>Mode 3</strong> is full resolution (240w x 160h) RBG15 color. You might not be used
-RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
+to RGB15, since modern computers have 24 or 32 bit colors. In RGB15, there's 5
-bits for each color channel, and the highest bit is totally ignored.</li>
+bits for each color channel stored within a <code>u16</code> value, and the highest bit is
+simply ignored.</li>
 <li><strong>Mode 4</strong> is full resolution paletted color. Instead of being a <code>u16</code> color, each
 pixel value is a <code>u8</code> palette index entry, and then the display uses the
 palette memory (which we'll talk about later) to store the actual color data.
@@ -513,9 +522,10 @@ reduced resolution to compensate (video memory is only so big!). The screen is
 effectively only 160w x 128h in this mode.</li>
 </ul>
 <a class="header" href="#cgb-mode" id="cgb-mode"><h2>CGB Mode</h2></a>
-<p>Bit 3 is read only. It's on if you're running in CGB mode. Since we're making
+<p>Bit 3 is effectively read only. Technically it can be flipped using a BIOS call,
-GBA games you'd think that it'll never be on at all, but I guess you can change
+but when you write to the display control normally it won't write to this bit,
-it with BIOS stuff. Still, basically not an important bit.</p>
+so we'll call it effectively read only.</p>
+<p>This bit is on if the CPU is in CGB mode.</p>
 <a class="header" href="#page-flipping" id="page-flipping"><h2>Page Flipping</h2></a>
 <p>Bit 4 lets you pick which page to use. This is only relevent in video modes 4 or
 5, and is just ignored otherwise. It's very easy to remember: when the bit is 0
@@ -554,6 +564,8 @@ some nifty graphical effects.</p>
 binary</a>, and we get
 <code>0b100_0000_0011</code>. So, that's setting Mode 3 with background 2 enabled and
 nothing else special.</p>
+<p>However, I think we can do better than that. This is a prime target for more
+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
@@ -569,15 +581,186 @@ trying to set an individual byte, and we also have to be careful if we use
 <code>memcopy</code> or <code>memset</code> as well, because they're byte oriented by default and
 don't know to follow the special rules.</p>
 <a class="header" href="#rgb15" id="rgb15"><h2>RGB15</h2></a>
-<p>TODO</p>
+<p>As I said before, RGB15 stores a color within a <code>u16</code> value using 5 bits for
+each color channel.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const RED:   u16 = 0b0_00000_00000_11111;
+pub const GREEN: u16 = 0b0_00000_11111_00000;
+pub const BLUE:  u16 = 0b0_11111_00000_00000;
+#}</code></pre></pre>
+<p>In Mode 3 and Mode 5 we write direct color values into VRAM, and in Mode 4 we
+write palette index values, and then the color values go into the PALRAM.</p>
 <a class="header" href="#mode-3" id="mode-3"><h2>Mode 3</h2></a>
-<p>TODO</p>
+<p>Mode 3 is pretty easy. We have a full resolution grid of rgb15 pixels. There's
+160 rows of 240 pixels each, with the base address being the top left corner. A
+particular pixel uses normal &quot;2d indexing&quot; math:</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+let row_five_col_seven = 5 + (7 * SCREEN_WIDTH);
+#}</code></pre></pre>
+<p>To draw a pixel, we just write a value at the address for the row and col that
+we want to draw to.</p>
 <a class="header" href="#mode-4" id="mode-4"><h2>Mode 4</h2></a>
-<p>TODO</p>
+<p>Mode 4 introduces page flipping. Instead of one giant page at <code>0x0600_0000</code>,
+there's Page 0 at <code>0x0600_0000</code> and then Page 1 at <code>0x0600_A000</code>. The resolution
+for each page is the same as above, but instead of writing <code>u16</code> values, the
+memory is treated as <code>u8</code> indexes into PALRAM. The PALRAM starts at
+<code>0x0500_0000</code>, and there's enough space for 256 palette entries (each a <code>u16</code>).</p>
+<p>To set the color of a palette entry we just do a normal <code>u16</code> write_volatile.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+(0x0500_0000 as *mut u16).offset(target_index).write_volatile(new_color)
+#}</code></pre></pre>
+<p>To draw a pixel we set the palette entry that we want the pixel to use. However,
+we must remember the &quot;minimum size&quot; write limitation that applies to VRAM. So,
+if we want to change just a single pixel at a time we must</p>
+<ol>
+<li>Read the full <code>u16</code> it's a part of.</li>
+<li>Clear the half of the <code>u16</code> we're going to replace</li>
+<li>Write the half of the <code>u16</code> we're going to replace with the new value</li>
+<li>Write that result back to the address.</li>
+</ol>
+<p>So, the math for finding a byte offset is the same as Mode 3 (since they're both
+a 2d grid). If the byte offset is EVEN it'll be the high bits of the <code>u16</code> at
+half the byte offset rounded down. If the offset is ODD it'll be the low bits of
+the <code>u16</code> at half the byte.</p>
+<p>Does that make sense?</p>
+<ul>
+<li>If we want to write pixel (0,0) the byte offset is 0, so we change the high
+bits of <code>u16</code> offset 0. Then we want to write to (1,0), so the byte offset is
+1, so we change the low bits of <code>u16</code> offset 0. The pixels are next to each
+other, and the target bytes are next to each other, good so far.</li>
+<li>If we want to write to (5,6) that'd be byte <code>5 + 6 * 240 = 1445</code>, so we'd
+target the low bits of <code>u16</code> offset <code>floor(1445/2) = 722</code>.</li>
+</ul>
+<p>As you can see, trying to write individual pixels in Mode 4 is mostly a bad
+time. Fret not! We don't <em>have</em> to write individual bytes. If our data is
+arranged correctly ahead of time we can just write <code>u16</code> or <code>u32</code> values
+directly. The video hardware doesn't care, it'll get along just fine.</p>
 <a class="header" href="#mode-5" id="mode-5"><h2>Mode 5</h2></a>
-<p>TODO</p>
+<p>Mode 5 is also a two page mode, but instead of compressing the size of a pixel's
+data to fit in two pages, we compress the resolution.</p>
+<p>Mode 5 is full <code>u16</code> color, but only 160w x 128h per page.</p>
 <a class="header" href="#in-conclusion-1" id="in-conclusion-1"><h2>In Conclusion...</h2></a>
-<p>TODO</p>
+<p>So what got written into VRAM in <code>hello1</code>?</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+    (0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+#}</code></pre></pre>
+<p>So at pixels <code>(120,80)</code>, <code>(136,80)</code>, and <code>(120,96)</code> we write three values. Once
+again we probably need to <a href="https://www.wolframalpha.com/">convert them</a> into
+binary to make sense of it.</p>
+<ul>
+<li>0x001F: 0b11111</li>
+<li>0x03E0: 0b11111_00000</li>
+<li>0x7C00: 0b11111_00000_00000</li>
+</ul>
+<p>Ah, of course, a red pixel, a green pixel, and a blue pixel.</p>
+<a class="header" href="#hello2" id="hello2"><h1>hello2</h1></a>
+<p>Okay so let's have a look again:</p>
+<p><code>hello1</code></p>
+<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
+#![no_std]
+
+#[cfg(not(test))]
+#[panic_handler]
+fn panic(_info: &amp;core::panic::PanicInfo) -&gt; ! {
+  loop {}
+}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
+  unsafe {
+    (0x04000000 as *mut u16).write_volatile(0x0403);
+    (0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+    loop {}
+  }
+}
+</code></pre></pre>
+<p>Now let's clean this up so that it's clearer what's going on.</p>
+<p>First we'll label that display control stuff:</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const DISPCNT: *mut u16 = 0x04000000 as *mut u16;
+pub const MODE3: u16 = 3;
+pub const BG2: u16 = 0b100_0000_0000;
+#}</code></pre></pre>
+<p>Next we make some const values for the actual pixel drawing</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const VRAM: usize = 0x06000000;
+pub const SCREEN_WIDTH: isize = 240;
+#}</code></pre></pre>
+<p>And then we want a small helper function for putting together a color value.</p>
+<p>Happily, this one can even be declared as a const function. At the time of
+writing, we've got the &quot;minimal const fn&quot; support in nightly. It really is quite
+limited, but I'm happy to let rustc and LLVM pre-compute as much as they can
+when it comes to the GBA's tiny CPU.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub const fn rgb16(red: u16, green: u16, blue: u16) -&gt; u16 {
+  blue &lt;&lt; 10 | green &lt;&lt; 5 | red
+}
+#}</code></pre></pre>
+<p>Finally, we'll make a function for drawing a pixel in Mode 3. Even though it's
+just a one-liner, having the &quot;important parts&quot; be labeled as function arguments
+usually helps you think about it a lot better.</p>
+<pre><pre class="playpen"><code class="language-rust">
+# #![allow(unused_variables)]
+#fn main() {
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}
+#}</code></pre></pre>
+<p>So now we've got this:</p>
+<p><code>hello2</code></p>
+<pre><pre class="playpen"><code class="language-rust">#![feature(start)]
+#![no_std]
+
+#[cfg(not(test))]
+#[panic_handler]
+fn panic(_info: &amp;core::panic::PanicInfo) -&gt; ! {
+  loop {}
+}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -&gt; isize {
+  unsafe {
+    DISPCNT.write_volatile(MODE3 | BG2);
+    mode3_pixel(120, 80, rgb16(31, 0, 0));
+    mode3_pixel(136, 80, rgb16(0, 31, 0));
+    mode3_pixel(120, 96, rgb16(0, 0, 31));
+    loop {}
+  }
+}
+
+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 fn rgb16(red: u16, green: u16, blue: u16) -&gt; u16 {
+  blue &lt;&lt; 10 | green &lt;&lt; 5 | red
+}
+
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}
+</code></pre></pre>
 
                     </main>
 

examples/hello2.rs

@@ -0,0 +1,34 @@
+#![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);
+    mode3_pixel(120, 80, rgb16(31, 0, 0));
+    mode3_pixel(136, 80, rgb16(0, 31, 0));
+    mode3_pixel(120, 96, rgb16(0, 0, 31));
+    loop {}
+  }
+}
+
+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 fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
+  blue << 10 | green << 5 | red
+}
+
+pub unsafe fn mode3_pixel(col: isize, row: isize, color: u16) {
+  (VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
+}

src/core_extras.rs

@@ -1,13 +1,13 @@
 //! Things that I wish were in core, but aren't.
 
-#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
-#[repr(transparent)]
 /// A simple wrapper to any `*mut T` so that the basic "read" and "write"
 /// operations are volatile.
 ///
 /// Accessing the GBA's IO registers and video ram and specific other places on
 /// **must** be done with volatile operations. Having this wrapper makes that
 /// more clear for all the global const values into IO registers.
+#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
+#[repr(transparent)]
 pub struct VolatilePtr<T>(pub *mut T);
 
 impl<T> core::fmt::Pointer for VolatilePtr<T> {

src/lib.rs

@@ -28,5 +28,5 @@ pub mod video_ram;
 
 /// Combines the Red, Blue, and Green provided into a single color value.
 pub const fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
-  red | green << 5 | blue << 10
+  blue << 10 | green << 5 | red
 }

src/video_ram.rs

@@ -28,6 +28,13 @@ pub const SCREEN_HEIGHT: isize = 160;
 /// value as just being a `usize`.
 pub const VRAM_BASE_ADDRESS: usize = 0x0600_0000;
 
+/// Draws a pixel to the screen while in Display Mode 3, with bounds checks.
+pub fn mode3_pixel(col: isize, row: isize, color: u16) {
+  assert!(col >= 0 && col < SCREEN_WIDTH);
+  assert!(row >= 0 && row < SCREEN_HEIGHT);
+  unsafe { mode3_pixel_unchecked(col, row, color) }
+}
+
 /// Draws a pixel to the screen while in Display Mode 3.
 ///
 /// Coordinates are relative to the top left corner.
@@ -39,13 +46,6 @@ pub const VRAM_BASE_ADDRESS: usize = 0x0600_0000;
 ///
 /// * `col` must be in `0..SCREEN_WIDTH`
 /// * `row` must be in `0..SCREEN_HEIGHT`
-pub unsafe fn mode3_plot_unchecked(col: isize, row: isize, color: u16) {
+pub unsafe fn mode3_pixel_unchecked(col: isize, row: isize, color: u16) {
   core::ptr::write_volatile((VRAM_BASE_ADDRESS as *mut u16).offset(col + row * SCREEN_WIDTH), color);
 }
-
-/// Draws a pixel to the screen while in Display Mode 3, with bounds checks.
-pub fn mode3_plot(col: isize, row: isize, color: u16) {
-  assert!(col >= 0 && col < SCREEN_WIDTH);
-  assert!(row >= 0 && row < SCREEN_HEIGHT);
-  unsafe { mode3_plot_unchecked(col, row, color) }
-}

thumbv4-none-agb.json

@@ -18,7 +18,7 @@
   "linker": "arm-none-eabi-ld",
   "linker-flavor": "ld",
   "linker-is-gnu": true,
-  "llvm-target": "thumbv4-nintendo-agb",
+  "llvm-target": "thumbv4-none-agb",
   "os": "none",
   "panic-strategy": "abort",
   "pre-link-args": {