BIOS:
There are several BIOSes inside the DS. First, there's the original GBA BIOS which is not accessible in DS mode. Then there's the DS ARM7 BIOS which resides at location 0x00000000 and is read-protected in a similar way as on the GBA. It also has an extra read security for the first 0x1205 bytes which is enabled at boot time. Then there's a ARM9 BIOS which is located at 0xFFFF0000. See the memory map for more details.
These BIOSes must not be confused with the Firmware which holds the boot menu and pictochat!