I/O List

If the I/O port (3n-1) returns 0, card is not there.

I/O port (3n-1) returns the card type.
	Top 4 bits mean the card genre:
		0x0: Video
		0x1: Audio
		0x2: System
	Bottom 4 bits mean the card type:
		Video:
			0x1: Video Mark I
			0x2: Video Mark II
		Audio:
			0x1: Sound Mark I
		System:
			0x1: Math Mark I
			0x2: Text Mark I
			0x3: Debug
			0x4: ROMcard Mark I

Video Mark I card maps itself to:
	+0: VDP address
	+1: VDP data
	+2: See above

Video Mark II card maps itself to:
	+0: VDG VRAM address
	+1: VDG VRAM data
	+2: See above

Sound Mark I card maps itself to:
	+0: PSG address
	+1: PSG data
	+2: See above

Math Mark I card maps itself to:
	+0: AM9511 stack
	+1: AM9511 command (W) / AM9511 ready (R)
	+2: See above

Text Mark I card maps itself to:
	+0: Keyboard data
	+1: Empty
	+2: See above

Debug card maps itself to:
	+0: Address
	+1: LED
		Bit to 7-segment map:
		 0
		1 2
		 3
		4 5
		 6 7
	+2: See above

ROMcard Mark I card maps itself to:
	+0: Address (Holds 24 bit address inside, gets left shifted by each write)
	+1: Data
	+2: See above