gba/src/asm_runtime.rs

//! This module holds the assembly runtime that supports your Rust program.
//!
//! Most importantly, you can set the [`RUST_IRQ_HANDLER`] variable to assign
//! which function should be run during a hardware interrupt.
//! * When a hardware interrupt occurs, control first goes to the BIOS, which
//!   will then call the assembly runtime's handler.
//! * The assembly runtime handler will properly acknowledge the interrupt
//!   within the system on its own without you having to do anything.
//! * If a function is set in the `RUST_IRQ_HANDLER` variable then that function
//!   will be called and passed the bits for which interrupt(s) occurred.

use crate::{
  dma::DmaControl,
  gba_cell::GbaCell,
  interrupts::IrqFn,
  mgba::MGBA_LOGGING_ENABLE_REQUEST,
  mmio::{DMA3_SRC, IME, MGBA_LOG_ENABLE},
};

/// The function pointer that the assembly runtime calls when an interrupt
/// occurs.
pub static RUST_IRQ_HANDLER: GbaCell<Option<IrqFn>> = GbaCell::new(None);

const DMA_32_BIT_MEMCPY: DmaControl =
  DmaControl::new().with_transfer_32bit(true).with_enabled(true);

const DMA3_OFFSET: usize = DMA3_SRC.as_usize() - 0x0400_0000;
const IME_OFFSET: usize = IME.as_usize() - 0x0400_0000;

#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
#[link_section = ".text.gba_rom_header"]
unsafe extern "C" fn __start() -> ! {
  core::arch::asm!(
    "b 1f",
    ".space 0xE0",
    "1:", /* post header */
    "mov r12, #{mmio_base}",
    "add r0, r12, #{waitcnt_offset}",
    "ldr r1, ={waitcnt_setting}",
    "strh r1, [r0]",

    /* iwram copy */
    "ldr r4, =__iwram_word_copy_count",
    bracer::when!("r4" != "#0" [label_id=1] {
      "add r3, r12, #{dma3_offset}",
      "mov r5, #{dma3_setting}",
      "ldr r0, =__iwram_start",
      "ldr r2, =__iwram_position_in_rom",
      "str r2, [r3]", /* source */
      "str r0, [r3, #4]", /* destination */
      "strh r4, [r3, #8]", /* word count */
      "strh r5, [r3, #10]", /* set control bits */
    }),

    /* ewram copy */
    "ldr r4, =__ewram_word_copy_count",
    bracer::when!("r4" != "#0" [label_id=1] {
      "add r3, r12, #{dma3_offset}",
      "mov r5, #{dma3_setting}",
      "ldr r0, =__ewram_start",
      "ldr r2, =__ewram_position_in_rom",
      "str r2, [r3]", /* source */
      "str r0, [r3, #4]", /* destination */
      "strh r4, [r3, #8]", /* word count */
      "strh r5, [r3, #10]", /* set control bits */
    }),

    /* bss zero */
    "ldr r4, =__bss_word_clear_count",
    bracer::when!("r4" != "#0" [label_id=1] {
      "ldr r0, =__bss_start",
      "mov r2, #0",
      "2:",
      "str r2, [r0], #4",
      "subs r4, r4, #1",
      "bne 2b",
    }),

    /* assign the runtime irq handler */
    "ldr r1, ={runtime_irq_handler}",
    "str r1, [r12, #-4]",

    /* ask for mGBA logging to be enabled. This should be harmless if we're not using mgba. */
    "ldr r0, ={mgba_log_enable}",
    "ldr r1, ={mgba_logging_enable_request}",
    "str r1, [r0]",

    /* call to rust main */
    "ldr r0, =main",
    "bx r0",
    // main shouldn't return, but if it does just SoftReset
    "swi #0",
    mmio_base = const 0x0400_0000,
    waitcnt_offset = const 0x204,
    waitcnt_setting = const 0x4317 /*sram8,r0:3.1,r1:4.2,r2:8.2,no_phi,prefetch*/,
    dma3_offset = const DMA3_OFFSET,
    dma3_setting = const DMA_32_BIT_MEMCPY.to_u16(),
    runtime_irq_handler = sym runtime_irq_handler,
    mgba_log_enable = const MGBA_LOG_ENABLE.as_usize(),
    mgba_logging_enable_request = const MGBA_LOGGING_ENABLE_REQUEST,
    options(noreturn)
  )
}

#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
#[link_section = ".iwram.runtime.irq.handler"]
unsafe extern "C" fn runtime_irq_handler() {
  // On Entry: r0 = 0x0400_0000 (mmio_base)
  core::arch::asm!(
    /* swap IME off, user can turn it back on if they want */
    "add r12, r0, #{ime_offset}",
    "mov r3, #0",
    "swp r3, r3, [r12]",

    /* Read/Update IE and IF */
    "ldr r0, [r12, #-8]",
    "and r0, r0, r0, LSR #16",
    "strh r0, [r12, #-6]",

    /* Read/Update BIOS_IF */
    "sub  r2, r12, #(0x208+8)",
    "ldrh r1, [r2]",
    "orr  r1, r1, r0",
    "strh r1, [r2]",

    /* Call the Rust fn pointer (if set), using System mode */
    "ldr r1, ={RUST_IRQ_HANDLER}",
    "ldr r1, [r1]",
    bracer::when!("r1" != "#0" [label_id=9] {
      bracer::with_spsr_held_in!("r2", {
        bracer::set_cpu_control!(System, irq_masked: false, fiq_masked: false),

        // Note(Lokathor): We are *SKIPPING* the part where we ensure that the
        // System stack pointer is aligned to 8 during the call to the rust
        // function. This is *technically* against the AAPCS ABI, but the GBA's
        // ARMv4T CPU does not even support any instructions that require an
        // alignment of 8. By not bothering to align the stack, we save about 5
        // cycles total. Which is neat, but if this were on the DS (which has an
        // ARMv5TE CPU) you'd want to ensure the aligned stack.

        bracer::with_pushed_registers!("{{r2, r3, r12, lr}}", {
          bracer::adr_lr_then_bx_to!(reg="r1", label_id=1)
        }),

        bracer::set_cpu_control!(Supervisor, irq_masked: true, fiq_masked: false),
      }),
    }),

    /* Restore initial IME setting and return */
    "swp r3, r3, [r12]",
    "bx lr",
    ime_offset = const IME_OFFSET,
    RUST_IRQ_HANDLER = sym RUST_IRQ_HANDLER,
    options(noreturn)
  )
}

// For now, the division fns can just keep living here.

/// Returns 0 in `r0`, while placing the `numerator` into `r1`.
///
/// This is written in that slightly strange way so that `div` function and
/// `divmod` functions can share the same code path.
///
/// See: [__aeabi_idiv0][aeabi-division-by-zero]
///
/// [aeabi-division-by-zero]: https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst#division-by-zero
#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
// this should literally never get called for real, so we leave it in ROM
extern "C" fn __aeabi_idiv0(numerator: i32) -> i32 {
  unsafe {
    core::arch::asm!(
      // this comment stops rustfmt from making this a one-liner
      "mov r1, r0",
      "mov r0, #0",
      "bx  lr",
      options(noreturn)
    )
  }
}

/// Returns `u32 / u32`
///
/// This implementation is *not* the fastest possible division, but it is
/// extremely compact.
///
/// See: [__aeabi_uidiv][aeabi-integer-32-32-division]
///
/// [aeabi-integer-32-32-division]:
///     https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst#integer-32-32-32-division-functions
#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
#[link_section = ".iwram.aeabi.uidiv"]
extern "C" fn __aeabi_uidiv(numerator: u32, denominator: u32) -> u32 {
  // Note(Lokathor): Other code in this module relies on being able to call this
  // function without affecting r12, so any future implementations of this code
  // **must not** destroy r12.
  unsafe {
    core::arch::asm!(
      // Check for divide by 0
      "cmp   r1, #0",
      "beq   {__aeabi_idiv0}",
      // r3(shifted_denom) = denom
      "mov   r3, r1",
      // while shifted_denom < (num>>1): shifted_denom =<< 1;
      "cmp   r3, r0, lsr #1",
      "2:",
      "lslls r3, r3, #1",
      "cmp   r3, r0, lsr #1",
      "bls   2b",
      // r0=quot(init 0), r1=denom, r2=num, r3=shifted_denom
      "mov   r2, r0",
      "mov   r0, #0",
      // subtraction loop
      "3:",
      "cmp   r2, r3",
      "subcs r2, r2, r3",
      "adc   r0, r0, r0",
      "mov   r3, r3, lsr #1",
      "cmp   r3, r1",
      "bcs   3b",
      "bx    lr",
      __aeabi_idiv0 = sym __aeabi_idiv0,
      options(noreturn)
    )
  }
}

/// Returns `i32 / i32`
///
/// See: [__aeabi_idiv][aeabi-integer-32-32-division]
///
/// [aeabi-integer-32-32-division]:
///     https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst#integer-32-32-32-division-functions
#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
#[link_section = ".iwram.aeabi.idiv"]
extern "C" fn __aeabi_idiv(numerator: i32, denominator: i32) -> u32 {
  unsafe {
    core::arch::asm!(
      // determine if `numerator` and `denominator` are the same sign
      "eor   r12, r1, r0",
      // convert both values to their unsigned absolute value.
      "cmp   r0, #0",
      "rsblt r0, r0, #0",
      "cmp   r1, #0",
      "rsclt r1, r1, #0",
      bracer::with_pushed_registers!("{{lr}}", {
        // divide them using `u32` division (this will check for divide by 0)
        "bl    {__aeabi_uidiv}",
      }),
      // if they started as different signs, flip the output's sign.
      "cmp   r12, #0",
      "rsblt r0, r0, #0",
      "bx    lr",
      __aeabi_uidiv = sym __aeabi_uidiv,
      options(noreturn)
    )
  }
}

/// Returns `(u32 / u32, u32 % u32)` in `(r0, r1)`.
///
/// The `u64` return value is a mild lie that gets Rust to grab up both the `r0`
/// and `r1` values when the function returns. If you transmute the return value
/// into `[u32; 2]` then you can separate the two parts of the return value, and
/// it will have no runtime cost.
///
/// See: [__aeabi_uidivmod][aeabi-integer-32-32-division]
///
/// [aeabi-integer-32-32-division]:
///     https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst#integer-32-32-32-division-functions
#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
#[link_section = ".iwram.aeabi.uidivmod"]
extern "C" fn __aeabi_uidivmod(numerator: u32, denominator: u32) -> u64 {
  unsafe {
    core::arch::asm!(
      // We need to save *both* input args until after the uidiv call. One of
      // them can be saved in `r12` because we know our uidiv doesn't actually
      // touch `r12`, while the other will be pushed onto the stack along with
      // `lr`. Since the function's output will be in `r0`, we push/pop `r1`.
      "mov   r12, r0",
      bracer::with_pushed_registers!("{{r1, lr}}", {
        "bl    {__aeabi_uidiv}",
      }),
      // Now r0 holds the `quot`, and we use it along with the input args to
      // calculate the `rem`.
      "mul   r2, r0, r1",
      "sub   r1, r12, r2",
      "bx    lr",
      __aeabi_uidiv = sym __aeabi_uidiv,
      options(noreturn)
    )
  }
}

/// Returns `(i32 / i32, i32 % i32)` in `(r0, r1)`.
///
/// The `u64` return value is a mild lie that gets Rust to grab up both the `r0`
/// and `r1` values when the function returns. If you transmute the return value
/// into `[i32; 2]` then you can separate the two parts of the return value, and
/// it will have no runtime cost.
///
/// See: [__aeabi_idivmod][aeabi-integer-32-32-division]
///
/// [aeabi-integer-32-32-division]:
///     https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst#integer-32-32-32-division-functions
#[naked]
#[no_mangle]
#[instruction_set(arm::a32)]
#[link_section = ".iwram.aeabi.idivmod"]
extern "C" fn __aeabi_idivmod(numerator: i32, denominator: i32) -> u64 {
  unsafe {
    core::arch::asm!(
      bracer::with_pushed_registers!("{{r4, r5, lr}}", {
        // store old numerator then make it the unsigned absolute
        "movs  r4, r0",
        "rsblt r0, r0, #0",
        // store old denominator then make it the unsigned absolute
        "movs  r5, r1",
        "rsblt r1, r1, #0",
        // divmod using unsigned.
        "bl    {__aeabi_uidivmod}",
        // if signs started opposite, quot becomes negative
        "eors  r12, r4, r5",
        "rsblt r0, r0, #0",
        // if numerator started negative, rem is negative
        "cmp   r4, #0",
        "rsblt r1, r1, #0",
      }),
      "bx    lr",
      __aeabi_uidivmod = sym __aeabi_uidivmod,
      options(noreturn)
    )
  }
}