Introduction and Limits chapters updated

Makefile.toml

@@ -43,10 +43,10 @@ fn main() -> std::io::Result<()> {
       std::process::Command::new("arm-none-eabi-objcopy").args(
         &["-O", "binary",
         &format!("target/thumbv4-none-agb/release/examples/{}",name),
-        &format!("target/example-{}.gba",name)])
+        &format!("target/{}.gba",name)])
         .output().expect("failed to objcopy!");
       std::process::Command::new("gbafix").args(
-        &[&format!("target/example-{}.gba",name)])
+        &[&format!("target/{}.gba",name)])
         .output().expect("failed to gbafix!");
     }
   }

README.md

@@ -8,17 +8,19 @@
 
 # gba
 
-A crate that helps you make GameBoy Advance (GBA) games
+This repository is both a [Tutorial Book](https://rust-console.github.io/gba/)
+that teaches you what you need to know to write Rust games for the GameBoy
+Advance (GBA), and also a [crate](https://crates.io/crates/gba) that you can
+use to do the same.
 
-# First Time Setup
+## First Time Setup
 
-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/00-introduction/03-development-setup.html) that goes with this
-crate.
+Writing a Rust program for the GBA requires a fair amount of special setup. All
+of the steps are detailed for you [in the Introduction chapter of the
+book](https://rust-console.github.io/gba/00-introduction/03-development-setup.html).
 
-If you've done the global setup once before and just want to get a new project
-started quickly we got you covered:
+If you've done the described global setup once before and just want to get a new
+project started quickly we got you covered:
 
 ```sh
 curl https://raw.githubusercontent.com/rust-console/gba/master/init.sh -sSf | bash -s APP_NAME

book/src/00-introduction/00-index.md

@@ -1,6 +1,6 @@
 # Introduction
 
-This is the book for learning how to write GBA games in Rust.
+This is the book for learning how to write GameBoy Advance (GBA) games in Rust.
 
 I'm **Lokathor**, the main author of the book. There's also **Ketsuban** who
 provides the technical advisement, reviews the PRs, and keeps my crazy in check.

book/src/00-introduction/04-hello-magic.md

@@ -21,8 +21,8 @@ goes:
 `hello_magic.rs`:
 
 ```rust
-#![feature(start)]
 #![no_std]
+#![feature(start)]
 
 #[panic_handler]
 fn panic(_info: &core::panic::PanicInfo) -> ! {

book/src/01-limitations/00-index.md

@@ -1 +1,10 @@
 # GBA Limitations
+
+There's a lot of seemingly normal Rust code that you can't write when compiling
+for the GBA.
+
+I mean there's a lot of stuff that you _can_ write, but we don't wanna get
+tripped up by assuming we can use something only to get stuck when it's missing.
+
+We'll start the book by reviewing all the major things we don't have so that we
+can avoid any surprises later.

book/src/01-limitations/01-no_floats.md

@@ -1 +0,0 @@
-# No Floats

book/src/01-limitations/01-no_std.md

@@ -0,0 +1,108 @@
+# No Std
+
+First up, as you already saw in the `hello_magic` code, we have to use the
+`#![no_std]` outer attribute on our program when we target the GBA. You can find
+some info about `no_std` in two official sources:
+
+* [unstable
+  book section](https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib)
+* [embedded
+  book section](https://rust-embedded.github.io/book/intro/no-std.html?highlight=no_std#a--no_std--rust-environment)
+
+The unstable book is borderline useless here because it's describing too many
+things in too many words. The embedded book is much better, but still fairly
+terse.
+
+## Bare Metal
+
+The GBA falls under what the Embedded Book calls "Bare Metal Environments".
+Basically, the machine powers on and immediately begins executing some ASM code.
+Our ASM startup was provided by `Ketsuban` (check the `crt0.s` file). We'll go
+over _how_ it works much later on, for now it's enough to know that it does
+work, and eventually controls passes into Rust code.
+
+On the rust code side of things, we determine our starting point with the
+`#[start]` attribute on our `main` function. The `main` function also has a
+specific type signature that's different from the usual `main` that you'd see in
+Rust. I'd tell you to read the unstable-book entry on `#[start]` but they
+[literally](https://doc.rust-lang.org/unstable-book/language-features/start.html)
+just tell you to look at the [tracking issue for
+it](https://github.com/rust-lang/rust/issues/29633) instead, and that's not very
+helpful either. Basically it just _has_ to be declared the way it is, even
+though there's nothing passing in the arguments and there's no place that the
+return value will go. The compiler won't accept it any other way.
+
+## No Standard Library
+
+The Embedded Book tells us that we can't use the standard library, but we get
+access to something called "libcore", which sounds kinda funny. What they're
+talking about is just [the core
+crate](https://doc.rust-lang.org/core/index.html), which is called `libcore`
+within the rust repository for historical reasons.
+
+The `core` crate is actually still a really big portion of Rust. The standard
+library doesn't actually hold too much code (relatively speaking), instead it
+just takes code form other crates and then re-exports it in an organized way. So
+with just `core` instead of `std`, what are we missing?
+
+In no particular order:
+
+* Allocation
+* Clock
+* Network
+* File System
+
+The allocation system and all the types that you can use if you have a global
+allocator are neatly packaged up in the
+[alloc](https://doc.rust-lang.org/alloc/index.html) crate. The rest isn't as
+nicely organized.
+
+It's _possible_ to implement a fair portion of the entire standard library
+within a GBA context and make the rest just panic if you try to use it. However,
+do you really need all that? Eh... probably not?
+
+* We don't need a file system, because all of our data is just sitting there in
+  the ROM for us to use. When programming we can organize our `const` data into
+  modules and such to keep it organized, but once the game is compiled it's just
+  one huge flat address space.
+* Networking, well, the GBA has a Link Cable you can use to communicate with
+  another GBA, but it's not really like a unix socket with TCP, so the standard
+  Rust networking isn't a very good match.
+* Clock is actually two different things at once. One is the ability to store
+  the time long term, which is a bit of hardware that some gamepaks have in them
+  (eg: pokemon ruby/sapphire/emerald). The GBA itself can't keep time while
+  power is off. However, the second part is just tracking time moment to moment,
+  which the GBA can totally do. We'll see how to access the timers soon enough.
+
+Which just leaves us with allocation. Do we need an allocator? Depends on your
+game. For demos and small games you probably don't need one. For bigger games
+you'll maybe want to get an allocator going eventually. It's in some sense a
+crutch, but it's a very useful one.
+
+So I promise that at some point we'll cover how to get an allocator going.
+Either a Rust Global Allocator (if practical), which would allow for a lot of
+the standard library types to be used "for free" once it was set up, or just a
+custom allocator that's GBA specific if Rust's global allocator style isn't a
+good fit for the GBA (I honestly haven't looked into it).
+
+## Bare Metal Panic
+
+TODO: expand this
+
+* Write `0xC0DE` to `0x4fff780` (`u16`) to enable mGBA logging. Write any other
+  value to disable it.
+* Read `0x4fff780` (`u16`) to check mGBA logging status.
+  * You get `0x1DEA` if debugging is active.
+  * Otherwise you get standard open bus nonsense values.
+* Write your message into the virtual `[u8; 255]` array starting at `0x4fff600`.
+  mGBA will interpret these bytes as a CString value.
+* Write `0x100` PLUS the message level to `0x4fff700` (`u16`) when you're ready
+  to send a message line:
+  * 0: Fatal (halts execution with a popup)
+  * 1: Error
+  * 2: Warning
+  * 3: Info
+  * 4: Debug
+* Sending the message also automatically zeroes the output buffer.
+* View the output within  the "Tools" menu, "View Logs...". Note that the Fatal
+  message, if any doesn't get logged.

book/src/01-limitations/02-core_only.md

@@ -1 +0,0 @@
-# Core Only

book/src/01-limitations/02-fixed_only.md

@@ -0,0 +1,13 @@
+# Fixed Only
+
+In addition to not having the standard library available, we don't even have a
+floating point unit available! We can't do floating pont math in hardware! We
+could still do floating point math as software computations if we wanted, but
+that's a slow, slow thing to do.
+
+Instead let's learn about another way to have fractional values called "Fixed
+Point"
+
+## Fixed Point
+
+TODO: describe fixed point, make some types, do the impls, all that.

book/src/01-limitations/03-volatile_destination.md

@@ -1,5 +1,7 @@
 # Volatile Destination
 
+TODO: replace all this one "the rant" is finalized
+
 There's a reasonable chance that you've never heard of `volatile` before, so
 what's that? Well, it's a term that can be used in more than one context, but
 basically it means "get your grubby mitts off my stuff you over-eager compiler".

book/src/SUMMARY.md

@@ -9,8 +9,8 @@
   * [Newtype](00-introduction/05-newtype.md)
   * [Help and Resources](00-introduction/06-help_and_resources.md)
 * [Limitations](01-limitations/00-index.md)
-  * [No Floats](01-limitations/01-no_floats.md)
-  * [Core Only](01-limitations/02-core_only.md)
+  * [No Std](01-limitations/01-no_std.md)
+  * [Fixed Only](01-limitations/02-fixed_only.md)
   * [Volatile Destination](01-limitations/03-volatile_destination.md)
 * [Concepts](02-concepts/00-index.md)
   * [CPU](02-concepts/01-cpu.md)

examples/hello_magic.rs

@@ -1,5 +1,5 @@
-#![feature(start)]
 #![no_std]
+#![feature(start)]
 
 #[panic_handler]
 fn panic(_info: &core::panic::PanicInfo) -> ! {

examples/mgba_panic_handler.rs

@@ -0,0 +1,109 @@
+#![no_std]
+#![feature(start)]
+
+#[no_mangle]
+pub unsafe extern "C" fn __clzsi2(mut x: usize) -> usize {
+  let mut y: usize;
+  let mut n: usize = 32;
+  y = x >> 16;
+  if y != 0 {
+    n = n - 16;
+    x = y;
+  }
+  y = x >> 8;
+  if y != 0 {
+    n = n - 8;
+    x = y;
+  }
+  y = x >> 4;
+  if y != 0 {
+    n = n - 4;
+    x = y;
+  }
+  y = x >> 2;
+  if y != 0 {
+    n = n - 2;
+    x = y;
+  }
+  y = x >> 1;
+  if y != 0 {
+    n - 2
+  } else {
+    n - x
+  }
+}
+
+#[panic_handler]
+fn panic(info: &core::panic::PanicInfo) -> ! {
+  unsafe {
+    const DEBUG_ENABLE_MGBA: *mut u16 = 0x4fff780 as *mut u16;
+    const DEBUG_OUTPUT_BASE: *mut u8 = 0x4fff600 as *mut u8;
+    const DEBUG_SEND_MGBA: *mut u16 = 0x4fff700 as *mut u16;
+    const DEBUG_SEND_FLAG: u16 = 0x100;
+    const DEBUG_FATAL: u16 = 0;
+    const DEBUG_ERROR: u16 = 1;
+    DEBUG_ENABLE_MGBA.write_volatile(0xC0DE);
+    if DEBUG_ENABLE_MGBA.read_volatile() == 0x1DEA {
+      // Give the location
+      if let Some(location) = info.location() {
+        let mut out_ptr = DEBUG_OUTPUT_BASE;
+        let line = location.line();
+        let mut line_bytes = [
+          (line / 10000) as u8,
+          ((line / 1000) % 10) as u8,
+          ((line / 1000) % 10) as u8,
+          ((line / 10) % 10) as u8,
+          (line % 10) as u8,
+        ];
+        for line_bytes_mut in line_bytes.iter_mut() {
+          *line_bytes_mut += b'0';
+        }
+        for b in "Panic: "
+          .bytes()
+          .chain(location.file().bytes())
+          .chain(", Line ".bytes())
+          .chain(line_bytes.iter().cloned())
+          .take(255)
+        {
+          out_ptr.write_volatile(b);
+          out_ptr = out_ptr.offset(1);
+        }
+      } else {
+        let mut out_ptr = DEBUG_OUTPUT_BASE;
+        for b in "Panic with no location info:".bytes().take(255) {
+          out_ptr.write_volatile(b);
+          out_ptr = out_ptr.offset(1);
+        }
+      }
+      DEBUG_SEND_MGBA.write_volatile(DEBUG_SEND_FLAG + DEBUG_ERROR);
+      // Give the payload
+      if let Some(payload) = info.payload().downcast_ref::<&str>() {
+        let mut out_ptr = DEBUG_OUTPUT_BASE;
+        for b in payload.bytes().take(255) {
+          out_ptr.write_volatile(b);
+          out_ptr = out_ptr.offset(1);
+        }
+      } else {
+        let mut out_ptr = DEBUG_OUTPUT_BASE;
+        for b in "no payload".bytes().take(255) {
+          out_ptr.write_volatile(b);
+          out_ptr = out_ptr.offset(1);
+        }
+      }
+      DEBUG_SEND_MGBA.write_volatile(DEBUG_SEND_FLAG + DEBUG_ERROR);
+      DEBUG_SEND_MGBA.write_volatile(DEBUG_SEND_FLAG + DEBUG_FATAL);
+    }
+  }
+  loop {}
+}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -> isize {
+  unsafe {
+    (0x400_0000 as *mut u16).write_volatile(0x0403);
+    (0x600_0000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
+    (0x600_0000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
+    (0x600_0000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
+    panic!("fumoffu!");
+  }
+}