ARM7 Registers

The 3DS utilizes an onboard ARM7 core to handle TWL_FIRM and AGB_FIRM's ARM7 requirements. This is due to the fact that much of the hardware used by both ARM7 and ARM9 is (evidently) not physically hooked up to ARM11. Thus, ARM11 cannot simply emulate ARM7.

This core seems to have the AGB BIOS available through hardware. This, combined with the onboard 2D graphics engine, makes for almost perfect Gameboy Advance "emulation."

Registers

ARM9 interfaces with the ARM7 through the following registers:

Name	Address	Width
ARM7_CNT	0x10018000	0x1
ARM7_CODE	0x10018080	?
ARM7_?_STATE	0x10018104	0x1
ARM7_?	0x10018108	0x2
ARM7_?	0x10018110	0x8?

ARM7_CNT

This indicates (controls?) the mode of the ARM7. 1 = TWL, 2 = AGB.

ARM7_CODE

This is the first code that will be run after execution begins. TwlProcess9 uses this to put ARM7 in a loop (TWL), and to set the POSTFLG and branch to more copied code (AGB).This doesn't seem to start execution by itself.

Memory map

The virtual memory mapping for the ARM7 is the same as for the other core. However, it has additional internal memory mapped to it. Interestingly enough, this memory all seems to lie within ARM9's own "internal memory."

• 0x8060000 -> 0x3800000 (ARM7-WRAM (64KB))
• 0x806C000 -> ?
• 0x80C0000 -> ? (0x10018104 is set to 1 before changing memory here, and 0 afterwards)
• 0x80BFE00 -> 0x3007E00 (CPU internal area, AGB boot code is written here)