Firmware

The Nintendo DS firmware is a 256KiB (512KiB for iQue) flash memory, that is not memory mapped. You can access it via SPI-bus.

Memory Access

Memory access is only possible via NDS7 with the SPICNT 0x40001c0 and the SPIDATA 0x40001c2 registers. DMA probably isn’t really feasable for reading or writing, because DMA has no 8-bit mode (only 16 or 32) and SPI has no functional 16-bit mode.