Hacking CulT - Research / Megadrive Programming / I/O ports

Research / Megadrive Programming / I/O ports

I/O

Registers

Address	Description
$A10001	Version
$A10003	Port A data
$A10005	Port B data
$A10007	Port C data
$A10009	Port A control
$A1000B	Port B control
$A1000D	Port C control
$A1000F	Port A TxData
$A10011	Port A RxData
$A10013	Port A serial control
$A10015	Port B TxData
$A10017	Port B RxData
$A10019	Port B serial control
$A1001B	Port C TxData
$A1001D	Port C RxData
$A1001F	Port C serial control

Reading even addresses returns the same as odd ones so you can use both $A10000 and $A10001.

$a10001 - version register

Reading version register returns a byte:

Bit 7 - Export bit. 1 if export (USA, Europe, etc.), 0 if domestic (Japan)

Bit 6 - Video type. 1 if PAL, 0 if NTSC

Bit 5 - 0 if Sega CD connected, 1 if not connected

Bit 4 - Unused (always 0)

Bits 3-0 - 0 - MegaDrive 1 / Genesis 1, 1 - newer. Newer have additional security hardware.

$a10003-7 - data registers

Reading a data register returns the state of all pins:

Bit 7 - Unused

Bit 6 - TH pin

Bit 5 - TR pin

Bit 4 - TL pin

Bit 3 - Data 3 pin

Bit 2 - Data 2 pin

Bit 1 - Data 1 pin

Bit 0 - Data 0 pin

Reading joypads

Here's the code:

pads_read:



		lea     ($FFFFFFF0).w,a0	;where the joypad states will be written

		lea     ($A10003).l,a1	;first joypad data port

		bsr.s   joypad_read	;do the first joypad

		addq.w  #2,a1		;do the second joypad

joypad_read:

		move.b  #0,(a1)		;set the first part

		nop     			;let the hardware parts act

		nop     

		move.b  (a1),d0		;move the lower byte to d0

		lsl.b   #2,d0		;shift some stuff

		andi.b  #$C0,d0		;exclude some dirty stuff

		move.b  #$40,(a1)		;set the second part

		nop     			;the delay

		nop     

		move.b  (a1),d1		;move the upper byte to d1

		andi.b  #$3F,d1		;exclude some dirty stuff

		or.b    d1,d0		;or d0 with d1

		not.b   d0		;not d0

		move.b  (a0),d1		;move the previous joypad state

		eor.b   d0,d1		;eor previous state with current state

					;(we get holded joypad state in d1)

		move.b  d0,(a0)+		;move the pressed state to ram

		and.b   d0,d1		;and stuff

		move.b  d1,(a0)+		;move the holded state to ram

		rts

After executing that code you will have:

$FFFFF0 - Current pressed state of joypad 1

$FFFFF1 - Current holded state of joypad 1

$FFFFF2 - Current pressed state of joypad 2

$FFFFF3 - Current holded state of joypad 2

The joypad output format:

Bit 7 - Start

Bit 6 - A button

Bit 5 - C button

Bit 4 - B button

Bit 3 - Right

Bit 2 - Left

Bit 1 - Down

Bit 0 - Up

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