libn3ds (fork)

libn3ds is a common library for 3DS ARM9/ARM11 bare metal projects.

See the original project here.

This fork adds support for a GBA cheat engine, enabling Action Replay (PARv3) cheat codes in open_agb_firm.

Changes

IPC command: IPC_CMD9_SET_GBA_CHEATS

A new IPC command (ipc_handler.h, ipc_handler.c) allows the ARM11 to send decoded cheat data to the ARM9. The ARM9 handler receives an array of cheat entries (up to LGY_MAX_CHEATS = 25) and stores them in global buffers (g_cheatData, g_cheatCount) for use during GBA boot.

ARM7 cheat engine (lgy7_code.s)

A new ARM7 IRQ handler (_gba_cheat_irq_handler) is added to the existing GBA stub code. When cheats are active, it replaces the BIOS IRQ dispatcher by patching the GBA IRQ vector. On each VBlank interrupt, the handler iterates a cheat table in IWRAM (0x03007ED0) and applies constant memory writes (8/16/32-bit), then forwards to the game's own IRQ handler.

Cheat engine installation (lgy9.c)

During setupBiosOverlay(), if cheats were received via IPC, the ARM9:

1. Copies the cheat engine code to GBA IWRAM at LGY9_CHEAT_ENGINE_LOC (0x03007E50), after the existing boot stub.
2. Writes the cheat count and entries to LGY9_CHEAT_DATA_LOC (0x03007ED0).
3. Patches the GBA IRQ vector (a7_vector[6..7]) to redirect to the cheat engine instead of the BIOS dispatcher.

Memory layout constants (lgy9.h)

New defines for the cheat engine and data table locations in GBA IWRAM:

Constant	GBA address	Description
LGY9_CHEAT_ENGINE_LOC	0x03007E50	Cheat engine code
LGY9_CHEAT_DATA_LOC	0x03007ED0	Cheat count + entry table

Public API (lgy_common.h)

• LGY_MAX_CHEATS (25) -- maximum cheat entries per game.
• Result LGY_setGbaCheats(const u32 *cheatData, u32 cheatCount) -- sends cheat data from ARM11 to ARM9 via IPC.