After a suggestion from [gdx] from MRC I took a look at the Rock'N'Roller (MSX1) and started to work on a patch for Ninja/Shinobi Tap multi controller adapter.

First step was to disassemble the game and figure out how the game deals with the controls. There are several calls to 3 of the BIOS routines that read the keyboard, the joysticks and the trigger buttons (from the joysticks) and decided that the best strategy was to emulate such BIOS routines and return correspondent values based on the buttons pressed on the controllers plugged in the multitap adapter.

Next step was to figure out a position in RAM out of the game execution to place the patch code -> address 0xdf00 to 0xe300 seemed nice

Following I have created the alternate routines:

• ALT_SNSMAT
• ALT_GTSTCK
• ALT_GTTRIG

The alternate routines fall back to the original BIOS call when the Ninja/Shinobi tap is not detected (a flag is reset) . for instance...

ALT_GTSTCK:
    ld b,a                  ; save row number
    ld a,(ninjaDetectFlag)  ; test for ninja presence
    and a                   ;
    ld a,b                  ; restore row number
    jp z,0d5h ; (GTSTCK)    ; if not present continue with BIOS routine
    ;
    ; read and return STICK value
...

Next step was to generate a list of addresses where the BIOS calls are called and write a quick and dirt and code to replace such calls with the alternate routines created. The +1 is the offset for the instruction code CD (call) at the addresses that call the bios routines.

; Patch Original game code 
    ld hl, ALT_SNSMAT  ; patch calls to 0141h (SNSMAT)
    ld (09648h+1),hl 
    ld (0966bh+1),hl 
    ld (09676h+1),hl 
    ld (09685h+1),hl 
    ld (09b7ch+1),hl 
...

Then I have created a function that will be called in the game main loop to scan the joystick ports and detect the presence of a multitap adapter om port A. If an adapter is detected, a flag is marked and the four taps are read and converted as a mirror of a keyboard matrix and joystick directionals and triggers.

NINJAPATCH:
	; check for presence of Tap
	call CHECKTAPS ; c = 0  0  0  0  a1 a0 b1 b0
	cpl            ;  c = 1  1  1  1 /a1 /a0 /b1 /b0
	and 08h        ; isolate bit a1 ->  a = 0 0 0 0 /a1 0 0 0
	ld (ninjaDetectFlag),a  ; /a1 should be 1 if a ninja or shinobi is detected
	ret z
	
; scan taps
;   ld de,0fa7ah  ;  Tap connected to joy port 2
   ld de,0ba3ah  ;   Tap connected to joy port 1
   ld hl,ntapData
   call GETNIN ; read TAPS on port 1	
   
; process taps *************************************
...

Button Mapping

Tap 1 is mapped as :

• Up, Down, Left, Right -> Ditto
• Trigger A - Z
• Trigger B - X
• Select  - SEL
• Start - Escape

That was done to allow full access and control of Options menu from the game, but it is still possible to access and control the Optiosn menu using the MSX keyboard.

Tap 2 is mapped as the QSCS (Q, S, Control, Shift) controls

Tap 3 is mapped as Joystick 1

Tap 4 is mapped as Joystick 2

To be continued...