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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" class = "active" > < strong aria-hidden = "true" > 3.5.< / strong > hello2< / a > < / li > < / ol > < / li > < / ol >
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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: & 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 >
< 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 " 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.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
pub const fn rgb16(red: u16, green: u16, blue: u16) -> u16 {
blue < < 10 | green < < 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 " important parts" 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: & 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);
}
< / code > < / pre > < / pre >
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< p > Exact same program that we started with, but much easier to read.< / p >
< 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 >
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