BIOS Functions

The GBA BIOS provides built-in routines accessible through software interrupts (SWI). stdgba wraps these in C++ functions, some of which are constexpr - the compiler evaluates them at compile time when possible and falls back to the BIOS call at runtime.

Common functions

Halting and waiting

#include <gba/bios>

// Wait for VBlank interrupt (most common - used every frame)
gba::VBlankIntrWait();

// Halt CPU until any interrupt
gba::Halt();

// Halt CPU until a specific interrupt
gba::IntrWait(true, { .vblank = true });

Math

// Square root (constexpr when argument is known at compile time)
auto root = gba::Sqrt(144u);  // 12

// Arc tangent
auto angle = gba::ArcTan2(dx, dy);

// Division (avoid - the compiler's division is usually better)
auto [quot, rem] = gba::Div(100, 7);

Memory copy

// CpuSet: 32-bit word copy/fill via BIOS
gba::CpuSet(src, dst, { .count = 256, .set_32bit = true });

// CpuFastSet: 32-bit copy in 8-word chunks (must be aligned, count multiple of 8)
gba::CpuFastSet(src, dst, { .count = 256 });

Note: For general memory copying, prefer standard memcpy/memset - stdgba’s optimised ARM assembly implementations are faster than the BIOS routines in most cases.

Decompression

// Decompress LZ77 data to work RAM (byte writes)
gba::LZ77UnCompWram(compressed_data, dest);

// Decompress LZ77 data to video RAM (halfword writes)
gba::LZ77UnCompVram(compressed_data, dest);

// Huffman decompression
gba::HuffUnCompReadNormal(compressed_data, dest);

// Run-length decompression
gba::RLUnCompReadNormalWrite8bit(compressed_data, dest);

Reset

// Soft reset (restart the ROM)
gba::SoftReset();  // [[noreturn]]

// Clear specific memory regions
gba::RegisterRamReset({
    .ewram = true,
    .iwram = true,
    .palette = true,
    .vram = true,
    .oam = true,
});

Constexpr BIOS functions

Several BIOS math functions are constexpr in stdgba. When called with compile-time arguments, the compiler evaluates them directly and embeds the result:

// Evaluated at compile time - no SWI at runtime
constexpr auto root = gba::Sqrt(256u);  // 16

// Evaluated at runtime - SWI 0x08
volatile unsigned int x = 256;
auto root2 = gba::Sqrt(x);  // BIOS call

This is possible because stdgba provides constexpr implementations of the algorithms alongside the SWI wrappers. The compiler chooses the appropriate path automatically.

tonclib comparison

The API names match the BIOS function names from the community documentation. The main difference is type safety: stdgba uses structs with named fields for configuration instead of raw integers with magic bit patterns.