gba/book/src/02-concepts/02-bios.md

BIOS

• Address Span: 0x0 to 0x3FFF (16k)

The BIOS of the GBA is a small read-only portion of memory at the very base of the address space. However, it is also hardware protected against reading, so if you try to read from BIOS memory when the program counter isn't pointed into the BIOS (eg: any time code you write is executing) then you get basically garbage data back.

So we're not going to spend time here talking about what bits to read or write within BIOS memory like we do with the other sections. Instead we're going to spend time talking about inline assembly (tracking issue) and then use it to call the GBA BIOS Functions.

Note that BIOS calls have more overhead than normal function calls, so don't go using them all over the place if you don't have to. They're also usually written more to be compact in terms of code than for raw speed, so you actually can out speed them in some cases. Between the increased overhead and not being as speed optimized, you can sometimes do a faster job without calling the BIOS at all. (TODO: investigate more about what parts of the BIOS we could potentially offer faster alternatives for.)

I'd like to take a moment to thank Marc Brinkmann (with contributions from Oliver Schneider and Philipp Oppermann) for writing this blog post. It's at least ten times the tutorial quality as the asm entry in the Unstable Book has. In fairness to the Unstable Book, the actual spec of how inline ASM works in rust is "basically what clang does", and that's specified as "basically what GCC does", and that's basically/shockingly not specified much at all despite GCC being like 30 years old.

So let's be slow and pedantic about this process.

Inline ASM

Fair Warning: Inline asm is one of the least stable parts of Rust overall, and if you write bad things you can trigger internal compiler errors and panics and crashes and make LLVM choke and die without explanation. If you write some inline asm and then suddenly your program suddenly stops compiling without explanation, try commenting out that whole inline asm use and see if it's causing the problem. Double check that you've written every single part of the asm call absolutely correctly, etc, etc.

Bonus Warning: The general information that follows regarding the asm macro is consistent from system to system, but specific information about register names, register quantities, asm instruction argument ordering, and so on is specific to ARM on the GBA. If you're programming for any other device you'll need to carefully investigate that before you begin.

Now then, with those out of the way, the inline asm docs describe an asm call as looking like this:

asm!(assembly template
   : output operands
   : input operands
   : clobbers
   : options
   );

And once you stick a lot of stuff in there it can absolutely be hard to remember the ordering of the elements. So we'll start with a code block that has some comments thrown in on each line:

asm!(/* ASM */ TODO
    :/* OUT */ TODO
    :/* INP */ TODO
    :/* CLO */ TODO
    :/* OPT */
);

Now we have to decide what we're gonna write. Obviously we're going to do some instructions, but those instructions use registers, and how are we gonna talk about them? We've got two choices.

1. We can pick each and every register used by specifying exact register names. In THUMB mode we have 8 registers available, named r0 through r7. If you switch into 32-bit mode there's additional registers that are also available.

2. We can specify slots for registers we need and let LLVM decide. In this style you name your slots $0, $1 and so on. Slot numbers are assigned first to all specified outputs, then to all specified inputs, in the order that you list them.

In the case of the GBA BIOS, each BIOS function has pre-designated input and output registers, so we will use the first style. If you use inline ASM in other parts of your code you're free to use the second style.

Assembly

This is just one big string literal. You write out one instruction per line, and excess whitespace is ignored. You can also do comments within your assembly using ; to start a comment that goes until the end of the line.

Assembly convention doesn't consider it unreasonable to comment potentially as much as every single line of asm that you write when you're getting used to things. Or even if you are used to things. This is cryptic stuff, there's a reason we avoid writing in it as much as possible.

Remember that our Rust code is in 16-bit mode. You can switch to 32-bit mode within your asm as long as you switch back by the time the block ends. Otherwise you'll have a bad time.

Outputs

A comma separated list. Each entry looks like

• "constraint" (binding)

An output constraint starts with a symbol:

• = for write only
• + for reads and writes
• & for for "early clobber", meaning that you'll write to this at some point before all input values have been read. It prevents this register from being assigned to an input register.

Followed by either the letter r (if you want LLVM to pick the register to use) or curly braces around a specific register (if you want to pick).

• The binding can be any single 32-bit or smaller value.
• If your binding has bit pattern requirements ("must be non-zero", etc) you are responsible for upholding that.
• If your binding type will try to Drop later then you are responsible for it being in a fit state to do that.
• The binding must be either a mutable binding or a binding that was pre-declared but not yet assigned.

Anything else is UB.

Inputs

This is a similar comma separated list.

• "constraint" (binding)

An input constraint doesn't have the symbol prefix, you just pick either r or a named register with curly braces around it.

• An input binding must be a single 32-bit or smaller value.
• An input binding should be a type that is Copy but this is not an absolute requirement. Having the input be read is semantically similar to using core::ptr::read(&binding) and forgetting the value when you're done.

Clobbers

Sometimes your asm will touch registers other than the ones declared for input and output.

Clobbers are declared as a comma separated list of string literals naming specific registers. You don't use curly braces with clobbers.

LLVM needs to know this information. It can move things around to keep your data safe, but only if you tell it what's about to happen.

Failure to define all of your clobbers can cause UB.

Options

There's only one option we'd care to specify. That option is "volatile".

Just like with a function call, LLVM will skip a block of asm if it doesn't see that any outputs from the asm were used later on. Nearly every single BIOS call (other than the math operations) will need to be marked as "volatile".

BIOS ASM

• Inputs are always r0, r1, r2, and/or r3, depending on function.
• Outputs are always zero or more of r0, r1, and r3.
• Any of the output registers that aren't actually used should be marked as clobbered.
• All other registers are unaffected.

All of the GBA BIOS calls are performed using the swi instruction, combined with a value depending on what BIOS function you're trying to invoke. If you're in 16-bit code you use the value directly, and if you're in 32-bit mode you shift the value up by 16 bits first.

Example BIOS Function: Division

For our example we'll use the division function, because GBATEK gives very clear instructions on how each register is used with that one:

Signed Division, r0/r1.
  r0  signed 32bit Number
  r1  signed 32bit Denom
Return:
  r0  Number DIV Denom ;signed
  r1  Number MOD Denom ;signed
  r3  ABS (Number DIV Denom) ;unsigned
For example, incoming -1234, 10 should return -123, -4, +123.
The function usually gets caught in an endless loop upon division by zero.

The math folks tell me that the r1 value should be properly called the "remainder" not the "modulus". We'll go with that for our function, doesn't hurt to use the correct names. Our Rust function has an assert against dividing by 0, then we name some bindings without giving them a value, we make the asm call, and then return what we got.

pub fn div_rem(numerator: i32, denominator: i32) -> (i32, i32) {
  assert!(denominator != 0);
  let div_out: i32;
  let rem_out: i32;
  unsafe {
    asm!(/* ASM */ "swi 0x06"
        :/* OUT */ "={r0}"(div_out), "={r1}"(rem_out)
        :/* INP */ "{r0}"(numerator), "{r1}"(denominator)
        :/* CLO */ "r3"
        :/* OPT */
    );
  }
  (div_out, rem_out)
}

I hope this all makes sense by now.

All The BIOS Functions

As for a full list of all the specific BIOS functions and their use, you should check the gba::bios module within the gba crate. There's just so many of them that enumerating them all here wouldn't serve much purpose.

Which is not to say that we'll never cover any BIOS functions in this book! Instead, we'll simply mention them when whenever they're relevent to the task at hand (such as sound or waiting for vblank).