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< title > hello1 - Rust GBA Guide< / title >
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< ol class = "chapter" > < li > < a href = "../introduction.html" > < strong aria-hidden = "true" > 1.< / strong > Introduction< / a > < / li > < li > < a href = "../ch00/index.html" > < strong aria-hidden = "true" > 2.< / strong > Ch 0: Development Setup< / a > < / li > < li > < a href = "../ch01/index.html" > < strong aria-hidden = "true" > 3.< / strong > Ch 1: Hello GBA< / a > < / li > < li > < ol class = "section" > < li > < a href = "../ch01/hello1.html" class = "active" > < strong aria-hidden = "true" > 3.1.< / strong > hello1< / a > < / li > < li > < a href = "../ch01/volatile.html" > < strong aria-hidden = "true" > 3.2.< / strong > Volatile< / a > < / li > < li > < a href = "../ch01/io_registers.html" > < strong aria-hidden = "true" > 3.3.< / strong > IO Registers< / a > < / li > < li > < a href = "../ch01/the_display_control_register.html" > < strong aria-hidden = "true" > 3.4.< / strong > The Display Control Register< / a > < / li > < li > < a href = "../ch01/video_memory_intro.html" > < strong aria-hidden = "true" > 3.5.< / strong > Video Memory Intro< / a > < / li > < li > < a href = "../ch01/hello2.html" > < strong aria-hidden = "true" > 3.6.< / strong > hello2< / a > < / li > < / ol > < / li > < li > < a href = "../ch02/index.html" > < strong aria-hidden = "true" > 4.< / strong > Ch 2: User Input< / a > < / li > < li > < ol class = "section" > < li > < a href = "../ch02/the_key_input_register.html" > < strong aria-hidden = "true" > 4.1.< / strong > The Key Input Register< / a > < / li > < li > < a href = "../ch02/the_vcount_register.html" > < strong aria-hidden = "true" > 4.2.< / strong > The VCount Register< / a > < / li > < li > < a href = "../ch02/light_cycle.html" > < strong aria-hidden = "true" > 4.3.< / strong > light_cycle< / a > < / li > < / ol > < / li > < li > < a href = "../ch03/index.html" > < strong aria-hidden = "true" > 5.< / strong > Ch 3: Memory and Objects< / a > < / li > < li > < ol class = "section" > < li > < a href = "../ch03/gba_memory_mapping.html" > < strong aria-hidden = "true" > 5.1.< / strong > GBA Memory Mapping< / a > < / li > < li > < a href = "../ch03/tile_data.html" > < strong aria-hidden = "true" > 5.2.< / strong > Tile Data< / a > < / li > < li > < a href = "../ch03/regular_backgrounds.html" > < strong aria-hidden = "true" > 5.3.< / strong > Regular Backgrounds< / a > < / li > < li > < a href = "../ch03/regular_objects.html" > < strong aria-hidden = "true" > 5.4.< / strong > Regular Objects< / a > < / li > < li > < a href = "../ch03/gba_rng.html" > < strong aria-hidden = "true" > 5.5.< / strong > GBA RNG< / a > < / li > < li > < a href = "../ch03/memory_game.html" > < strong aria-hidden = "true" > 5.6.< / strong > memory_game< / a > < / li > < / ol > < / li > < / ol >
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< a class = "header" href = "#hello1" id = "hello1" > < h1 > hello1< / h1 > < / a >
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< p > Our first example will be a totally minimal, full magic number crazy town.
Ready? Here goes:< / p >
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< p > < code > hello1.rs< / code > < / p >
< pre > < pre class = "playpen" > < code class = "language-rust" > #![feature(start)]
#![no_std]
#[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 {}
}
}
< / code > < / pre > < / pre >
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< p > Throw that into your project skeleton, build the program (as described back in
Chapter 0), and give it a run in your emulator. You should see a red, green, and
blue dot close-ish to the middle of the screen. If you don't, something already
went wrong. Double check things, phone a friend, write your senators, try asking
Ketsuban on the < a href = "https://discordapp.com/invite/aVESxV8" > Rust Community Discord< / a > ,
until you're able to get your three dots going.< / p >
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< a class = "header" href = "#a-basic-hello1-explanation" id = "a-basic-hello1-explanation" > < h2 > A basic hello1 explanation< / h2 > < / a >
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< p > So, what just happened? Even if you're used to Rust that might look pretty
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strange. We'll go over most of the little parts right here, and then bigger
parts will get their own sections.< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#![feature(start)]
#fn main() {
#}< / code > < / pre > < / pre >
< p > This enables the < a href = "https://doc.rust-lang.org/beta/unstable-book/language-features/start.html" > start
feature< / a > ,
which you would normally be able to read about in the unstable book, except that
the book tells you nothing at all except to look at the < a href = "https://github.com/rust-lang/rust/issues/29633" > tracking
issue< / a > .< / p >
< p > Basically, a GBA game is even more low-level than the < em > normal< / em > amount of
low-level that you get from Rust, so we have to tell the compiler to account for
that by specifying a < code > #[start]< / code > , and we need this feature on to do that.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#![no_std]
#fn main() {
#}< / code > < / pre > < / pre >
< p > There's no standard library available on the GBA, so we'll have to live a core
only life.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
#[panic_handler]
fn panic(_info: & core::panic::PanicInfo) -> ! {
loop {}
}
#}< / code > < / pre > < / pre >
< p > This sets our < a href = "https://doc.rust-lang.org/nightly/nomicon/panic-handler.html" > panic
handler< / a > .
Basically, if we somehow trigger a panic, this is where the program goes.
However, right now we don't know how to get any sort of message out to the user
so... we do nothing at all. We < em > can't even return< / em > from here, so we just sit in
an infinite loop. The player will have to reset the universe from the outside.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" > #[start]
fn main(_argc: isize, _argv: *const *const u8) -> isize {
< / code > < / pre > < / pre >
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< p > This is our < code > #[start]< / code > . We call it < code > main< / code > , but it's not like a < code > main< / code > that you'd
see in a Rust program. It's < em > more like< / em > the sort of < code > main< / code > that you'd see in a C
program, but it's still < strong > not< / strong > that either. If you compile a < code > #[start]< / code > program
for a target with an OS such as < code > arm-none-eabi-nm< / code > you can open up the debug
info and see that your result will have the symbol for the C < code > main< / code > along side
the symbol for the start < code > main< / code > that we write here. Our start < code > main< / code > is just its
own unique thing, and the inputs and outputs have to be like that because that's
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 " return to" when it's done.< / p >
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< p > Side note: if you want to learn more about stuff " before main gets called" 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 >
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< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
unsafe {
#}< / code > < / pre > < / pre >
< p > I hope you're all set for some < code > unsafe< / code > , because there's a lot of it to be had.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
(0x04000000 as *mut u16).write_volatile(0x0403);
#}< / code > < / pre > < / pre >
< p > Sure!< / 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 > Ah, of course.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
loop {}
}
}
#}< / code > < / pre > < / pre >
< p > And, as mentioned above, there's no place for a GBA program to " return to" , so
we can't ever let < code > main< / code > try to return there. Instead, we go into an infinite
< code > loop< / code > that does nothing. The fact that this doesn't ever return an < code > isize< / code >
value doesn't seem to bother Rust, because I guess we're at least not returning
any other type of thing instead.< / p >
< p > Fun fact: unlike in C++, an infinite loop with no side effects isn't Undefined
Behavior for us rustaceans... < em > semantically< / em > . In truth LLVM has a < a href = "https://github.com/rust-lang/rust/issues/28728" > known
bug< / a > in this area, so we won't
actually be relying on empty loops in any future programs.< / p >
< a class = "header" href = "#all-those-magic-numbers" id = "all-those-magic-numbers" > < h2 > All Those Magic Numbers< / h2 > < / a >
< p > Alright, I cheated quite a bit in the middle there. The program works, but I
didn't really tell you why because I didn't really tell you what any of those
magic numbers mean or do.< / p >
< ul >
< li > < code > 0x04000000< / code > is the address of an IO Register called the Display Control.< / li >
< li > < code > 0x06000000< / code > is the start of Video RAM.< / li >
< / ul >
< p > So we write some magic to the display control register once, then we write some
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other magic to three magic locations in the Video RAM. Somehow that shows three
dots. Gotta read on to find out why!< / p >
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