Programmer's Reference:

SDX
SDX cartridge consist of one register - combined Bank Select and Control register (0xD5E0 - WRITE ONLY), bit definition are as follows:
D7-D6 - Cartridge Control:

• 0x - SDX ON, External Cartridge (EXT) OFF
• 10 - SDX OFF, EXT ON
• 11 - SDX OFF, EXT OFF

D5-D0 - Bank number

EXT
External Cartridge register (D5E4 - WRITE ONLY):

D7 - External Cartridge enable (0) or disable (1)
D6 - for future use, should be written 0
D5-D0 - Bank number
note: D5 controls highest address line of 4mbit flash chip, and due to the memory map of this cartridge as a whole, is inverted - this means that space is shared between SDX and EXT carts (50/50%) SDX being first (lower 32 banks), and EXT being second (upper 32 banks)

RTC
RTC SPI PORT (D5E2 - RW):

D7-D4 - reserved, should be written 0
D3 (RO) - Serial In
D2 (WO) - Serial Out
D1 (WO) - Clock
D0 (WO) - Chip Enable
note: device used: DS1305

Hard drive
ATA register base: D5F0

Hard Drive Reset (D5F8 - WRITE ONLY):
D7-D1 - reserved, should be written 0
D0 - Hard drive reset (1) / normal operation (0)
note: software should ensure this pulse is at least 400ms long

Cartridge state control
STATE (D5FC - READ ONLY):
will read "S" (0x53) if in SDX is enabled, " " (0x20) when disabled
note: older version of cartridge will read "S" regardless if SDX is enabled or disabled

ID (D5FD-D5FF - READ ONLY):
reads "IDE" (0x49 0x44 0x45)