newtype_enum, and other updates

book/src/gba-asm.md

@@ -9,96 +9,102 @@ sometimes. Accordingly, you should know how assembly works on the GBA.
   `ARMv4` ISA, the `ARMv4T` variant, and specifically the `ARM7TDMI`
   microarchitecture. Someone at ARM decided that having both `ARM#` and `ARMv#`
   was a good way to [version things](https://en.wikichip.org/wiki/arm/versions),
-  even when the numbers don't match, and the rest of us have been sad ever
-  since. The link there will take you to the correct book within the big pile of
-  ARM books available within the ARM Infocenter. Note that there is also a [PDF
+  even when the numbers don't match. The rest of us have been sad ever since.
+  The link there will take you to the correct book specific to the GBA's
+  microarchitecture. There's a whole big pile of ARM books available within the
+  ARM Infocenter, so if you just google it or whatever make sure you end up
+  looking at the correct one. Note that there is also a [PDF
   Version](http://infocenter.arm.com/help/topic/com.arm.doc.ddi0210c/DDI0210B.pdf)
   of the documentation available, if you'd like that.
 
 * The [GBATek: ARM CPU
   Overview](https://problemkaputt.de/gbatek.htm#armcpuoverview) also has quite a
-  bit of info. Most of it is somewhat a duplication of what you'd find in the
-  ARM Infocenter reference manual, but it's also somewhat specialized towards
-  the GBA's specifics. It's in the usual, uh, "sparse" style that GBATEK is
-  written in, so I wouldn't suggest that read it first.
+  bit of info. Some of it is a duplication of what you'd find in the ARM
+  Infocenter reference manual. Some of it is specific to the GBA's chip. Some of
+  it is specific to the ARM chips within the DS and DSi. It's a bit of a jumbled
+  mess, and as with the rest of GBATEK, the explanations are in a "sparse" style
+  (to put it nicely), so I wouldn't take it as your only source.
 
 * The [Compiler Explorer](https://rust.godbolt.org/z/ndCnk3) can be used to
-  quickly look at assembly output of your Rust code. That link there will load
+  quickly look at assembly versions of your Rust code. That link there will load
   up an essentially blank `no_std` file with `opt-level=3` set and targeting
-  `thumbv6m-none-eabi`. That's _not_ the same as the GBA (it's two ISA revisions
-  later, ARMv6 instead of ARMv4), but it's the closest CPU target that ships
-  with rustc, so it's the closest you can get with the compiler explorer
-  website. If you're very dedicated I suppose you could setup a [local
+  `thumbv6m-none-eabi`. That's _not_ the same target as the GBA (it's two ISA
+  revisions later, ARMv6 instead of ARMv4), but it's the closest CPU target that
+  is bundled with rustc, so it's the closest you can get with the compiler
+  explorer website. If you're very dedicated I suppose you could setup a [local
   instance](https://github.com/mattgodbolt/compiler-explorer#running-a-local-instance)
   of compiler explorer and then add the extra target definition and so on, but
   that's _probably_ overkill.
 
-## ARM and THUMB
+## ARM and Thumb
 
 The "T" part in `ARMv4T` and `ARM7TDMI` means "Thumb". An ARM chip that supports
-Thumb mode has two different instruction sets instead of just one. The chip can
-run in ARM mode with 32-bit instructions, or it can run in THUMB mode with
-16-bit instructions. Apparently these modes are sometimes called `a32` and `t32`
-in a more modern context, but I will stick with ARM and THUMB because that's
-what other GBA references use (particularly GBATEK), and it's probably best to
-be more in agreement with them than with stuff for Raspberry Pi programming or
-whatever other modern ARM thing.
+Thumb has two different instruction sets instead of just one. The chip can run
+in ARM state with 32-bit instructions, or it can run in Thumb state with 16-bit
+instructions. Note that the CPU _state_ (ARM or Thumb) is distinct from the
+_mode_ (User, FIQ, IRQ, etc). Apparently these states are sometimes called
+`a32` and `t32` in a more modern context, but I will stick with ARM and Thumb
+because that's what the official ARM7TDMI manual and GBATEK both use.
 
-On the GBA, the memory bus that physically transfers data from the game pak into
+On the GBA, the memory bus that physically transfers data from the cartridge into
 the device is a 16-bit memory bus. This means that if you need to transfer more
 than 16 bits at a time you have to do more than one transfer. Since we'd like
 our instructions to get to the CPU as fast as possible, we compile the majority
-of our program with the THUMB instruction set. The ARM reference says that with
-THUMB instructions on a 16-bit memory bus system you get about 160% performance
+of our program with the Thumb instruction set. The ARM reference says that with
+Thumb instructions on a 16-bit memory bus system you get about 160% performance
 compared to using ARM instructions. That's absolutely something we want to take
-advantage of. Also, your THUMB compiled code is about 65% of the same code
+advantage of. Also, your Thumb compiled code is about 65% of the same code
 compiled with ARM. Since a game ROM can only be 32MB total, and we're trying to
 fit in images and sound too, we want to get space savings where we can.
 
-You may wonder, why is the THUMB code 65% as large if the instructions
-themselves are 50% as large, and why have ARM mode at all if there's such a
-benefit to be had with THUMB? Well, THUMB mode doesn't support as many different
-instructions as ARM mode does. Some lines of source code that can compile to a
-single ARM instruction might need to compile into more than one THUMB
-instruction. THUMB still has most of the really good instructions available, so
+You may wonder, why is the Thumb code 65% as large if the instructions
+themselves are 50% as large, and why have ARM state at all if there's such a
+benefit to be had with Thumb? Well, Thumb state doesn't support as many different
+instructions as ARM state does. Some lines of source code that can compile to a
+single ARM instruction might need to compile into more than one Thumb
+instruction. Thumb still has most of the really good instructions available, so
 it all averages out to about 65%.
 
-That said, some parts of a GBA program _must_ be written in ARM mode. Also, ARM
-mode does allow that increased instruction flexibility. So we _need_ to use ARM
-some of the time, and we might just _want_ to use ARM even when we don't need
-to. It is possible to switch modes on the fly, there's extremely minimal
-overhead, even less than doing some function calls. The only problem is the
-16-bit memory bus of the game pak giving us a needless speed penalty with our
-ARM code. The CPU _executes_ the ARM instructions at full speed, but then it has
-to wait while more instructions get sent in. What do we do? Well, code is
-ultimately just a different kind of data. We can copy parts of our code off the
-game pak ROM and place it into a part of the RAM that has a 32-bit memory bus.
-Then the CPU can execute the code from there, going at full speed. Of course,
-there's only a very small amount of RAM compared to the size of a game pak, so
-we'll only do this with a few select functions. Exactly which functions will
-probably depend on your game.
+That said, some parts of a GBA program _must_ be written for ARM state. Also,
+ARM state does allow that increased instruction flexibility. So we _need_ to use
+ARM some of the time, and we might just _want_ to use ARM even when we don't
+need to at other times. It is possible to switch states on the fly, there's
+extremely minimal overhead, even less than doing some function calls. The only
+problem is the 16-bit memory bus of the cartridge giving us a needless speed
+penalty with our ARM code. The CPU _executes_ the ARM instructions at full
+speed, but then it has to wait while more instructions get sent in. What do we
+do? Well, code is ultimately just a different kind of data. We can copy parts of
+our code off the cartridge ROM and place it into a part of the RAM that has a
+32-bit memory bus. Then the CPU can execute the code from there, going at full
+speed. Of course, there's only a very small amount of RAM compared to the size
+of a cartridge, so we'll only do this with a few select functions. Exactly which
+functions will probably depend on your game.
 
-One problem with this process is that Rust doesn't currently offer a way to mark
-individual functions for being ARM or THUMB. The whole program is compiled in a
-single mode. That's not an automatic killer, since we can use the `asm!` macro
-to write some inline assembly, then within our inline assembly we switch from
-THUMB to ARM, do some ARM stuff, and switch back to THUMB mode before the inline
-assembly is over. Rust is none the wiser to what happened. Yeah, it's clunky,
-that's why [it's on the 2019
-wishlist](https://github.com/rust-embedded/wg/issues/256#issuecomment-439677804)
-to fix it (then LLVM can manage it automatically for you).
+There's two problems that we face as Rust programmers:
 
-The bigger problem is that when we do that all of our functions still start off
-in THUMB mode, even if they temporarily use ARM mode. For the few bits of code
-that must start _already in_ ARM mode, we're stuck. Those parts have to be
-written in external assembly files and then included with the linker. We were
-already going to write some assembly, and we already use more than one file in
-our project all the time, those parts aren't a big problem. The big problem is
-that using custom linker scripts isn't transitive between crates.
+1) Rust offers no way to specify individual functions as being ARM or Thumb. The
+   whole program is compiled for one state or the other. Obviously this is no
+   good, so it's on the [2019 embedded
+   wishlist](https://github.com/rust-embedded/wg/issues/256#issuecomment-439677804),
+   and perhaps a fix will come.
+
+2) Rust offers no way to get a pointer to a function as well as the length of
+   the compiled function, so we can't copy a function from the ROM to some other
+   location because we can't even express statements about the function's data.
+   I also put this [on the
+   wishlist](https://github.com/rust-embedded/wg/issues/256#issuecomment-450539836),
+   but honestly I have much less hope that this becomes a part of rust.
+
+What this ultimately means is that some parts of our program have to be written
+in external assembly files and then added to the program with the linker. We
+were already going to write some assembly, and we already use more than one file
+in our project all the time, those parts aren't a big problem. The big problem
+is that using custom linker scripts to get assembly code into our final program
+isn't transitive between crates.
 
 What I mean is that once we have a file full of custom assembly that we're
 linking in by hand, that's not "part of" the crate any more. At least not as
-`cargo` see it. So we can't just upload it to `crates.io` and then depend on it
+`cargo` sees it. So we can't just upload it to `crates.io` and then depend on it
 in other projects and have `cargo` download the right version and and include it
 all automatically. We're back to fully manually copying files from the old
 project into the new one, adding more lines to the linker script each time we

src/ewram.rs

@@ -0,0 +1 @@
+//! Module for External Work RAM (`EWRAM`).

src/io/display.rs

@@ -49,10 +49,9 @@ impl DisplayControlSetting {
   }
 }
 
-/// The six display modes available on the GBA.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-#[repr(u16)]
-pub enum DisplayMode {
+newtype_enum! {
+  /// The six display modes available on the GBA.
+  DisplayMode = u16,
   /// * Affine: No
   /// * Layers: 0/1/2/3
   /// * Size(px): 256x256 to 512x512

src/iwram.rs

@@ -0,0 +1 @@
+//! Module for Internal Work RAM (`IWRAM`).

src/lib.rs

@@ -42,7 +42,7 @@ pub(crate) use gba_proc_macro::phantom_fields;
 /// }
 /// newtype! {
 ///   /// You can't derive most stuff above array size 32, so we add
-///   /// the `, no frills` modifier.
+///   /// the `, no frills` modifier to this one.
 ///   BigArray, [u8; 200], no frills
 /// }
 /// ```
@@ -67,6 +67,26 @@ macro_rules! newtype {
   };
 }
 
+/// Assists in defining a newtype that's an enum.
+///
+/// First give `NewType = OldType,`, then define the tags and their explicit
+/// values with zero or more entries of `TagName = base_value,`. In both cases
+/// you can place doc comments or other attributes directly on to the type
+/// declaration or the tag declaration.
+///
+/// The generated enum will get an appropriate `repr` attribute as well as Debug, Clone, Copy,
+///
+/// Example:
+/// ```
+/// newtype_enum! {
+///   /// The Foo
+///   Foo = u16,
+///   /// The Bar
+///   Bar = 0,
+///   /// The Zap
+///   Zap = 1,
+/// }
+/// ```
 #[macro_export]
 macro_rules! newtype_enum {
   (
@@ -86,21 +106,14 @@ macro_rules! newtype_enum {
   };
 }
 
-newtype_enum! {
-  /// the Foo
-  Foo = u16,
-  /// the Bar
-  Bar = 0,
-  /// The Zap
-  Zap = 1,
-}
-
 pub mod base;
 pub(crate) use self::base::*;
 
 pub mod bios;
 
-pub mod wram;
+pub mod iwram;
+
+pub mod ewram;
 
 pub mod io;
 

src/wram.rs

@@ -1 +0,0 @@
-//! Module for things related to WRAM.