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.

ARM7 has the AGB BIOS implemented in hardware. The BIOS is completely identical to the original AGB BIOS. The system is booted silently by calling SWI 0x1 (RegisterRamReset), followed by jumping to the code that does SWI 0x0 (SoftReset) to finish booting. The boot splash is still in BIOS, however, and can be seen by calling (or replacing one of the previous interrupts with) SWI 0x26 (HardReset).

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)