Displaying credits

The system ROM takes care of displaying the credits on the LED displays itself (see MV-LED board).

The credit count for each player is stored in $D00034 for player 1, and $D00035 for player 2. These locations are in the backup RAM, to keep the credit counts in case of a power failure during play.

It's up to the game to display the counts on the screen when needed. SNK recommends using a 8x8 fix layer font.

Hex to BCD conversion (untested):

moveq.l #0,d0        ; Clear D0
move.b  $D00034,d0   ; Read P1 credits (hex)
move.w  d0,d1        ; Make a copy
divu.w  #10,d0       ; Credits / 10
swap    d0           ; Take care of remainder first
sub.b   d1,d0        ; Get units
move.b  d0,d1
andi.b  #$0F,d1      ; Mask off tens
swap    d0           ; Go back to quotient
lsl.b   #4,d0        ; Shift tens to left digit
or.b    d1,d0        ; Mix tens and units

If $0C is in $D00034 we get $12, which is $0C in decimal but stored as hex. It's now trivial to get fix tile numbers for display on the fix layer.

move.w  #$7088,REG_VRAMADDR    ; Set position in fix map, top left
move.w  d0,d1                  ; Make a copy of the BCD value
move.w  #$20,REG_VRAMMOD       ; Auto-inc fix column
lsr.b   #4,d1                  ; Get tens
andi.w  #$000F,d1              ; Mask off eventual garbage
ori.w   #$5030,d1              ; Set palette 5 and tile offset $30 (ASCII "0")
move.w  d1,REG_VRAMRW          ; Write tens to VRAM
andi.w  #$000F,d0              ; Get units
ori.w   #$5030,d1              ; Same
move.w  d1,REG_VRAMRW          ; Write units to VRAM