kauthor47 / vstetrisfreeplay Goto Github PK

Contains patches to apply to ROMs for Nintendo Vs. Tetris to add a
free play option. This is controlled by DIP 5 (unused in the original
game). DIP 5 on is free play, DIP 5 off is normal coin mode. The following
is the "source" to the patches. I did not use an assembler, choosing to
instead hand-assemble since it was such a trivial patch. Code is commented
as well. To use this, apply the hex changes to the correct ROMs (for instance,
0xE350 is ROM e000.6a in MAME offset 0x350, 0xA027 is ROM a000.6c in MAME offset
0x27) and burn them using an EEPROM writer. If you would like, you can check
out <https://github.com/DragonMinded/arcadeutils> to use the `bindiff` utility
to apply the patches found in the two `.patch` files instead of performing
the edits manually.

draw_freeplay (0xE350)
; Stolen mostly from the credits display function. This
; draws "FREE PLAY" centered on the screeen in place of
; the credits display. There is no room for a space
; but fortunately each character has an accompanying X
; offset so we can position the text with a space.
A0 00    ; LDY #0x0 (Y register is the count)
A2 30    ; LDX #0x30 (X register is the char buffer offset)
B9 75 E3 ; LDA freeplay_text[count] (Grab an ascii character)
9D 01 04 ; STA 0x431[count * 4] (Place it in the char buffer offset)
A9 DF    ; LDA #0xDF (Grab the hardcoded Y offset)
38       ; SEC
9D 00 04 ; STA 0x430[count * 4] (Store it in the char buffer Y offset)
B9 7D E3 ; LDA freeplay_positions[count] (Grab the X offset for this char)
9D 03 04 ; STA 0x433[count * 4] (Store it in the char buffer X offset)
E8       ; INX (Increment X by 4 to get to next char buffer location)
E8       ; INX
E8       ; INX
E8       ; INX
C8       ; INY (Add one to the count)
C0 08    ; CPY #0x8 (Compare against 8, we do the above loop 8 times for "FREEPLAY")
D0 E5    ; BNE -27 (Jump back to the next ascii character grab.
A9 32    ; LDA #0x32
8D 30 05 ; STA 0x530 (Force the credits to "2" so that 1P/2P start works).
60       ; RTS (Back to the main game loop).

freeplay_text (0xE375)
46 52 45 45 50 4C 41 59

freeplay_positions (0xE37D)
5D 65 6D 75 85 8D 95 9D

draw_freeplay_or_credit (0xE385)
; This just jumps to either the above freeplay text function, or
; the original credits display function.
AD 17 40  ; LDA dipsw2_in
29 10     ; AND 0x10 (check DIP 5 is on)
D0 03     ; BNE 3
4C 46 AE  ; JMP 0xAE46 (old credit draw function)
4C 50 E3  ; JMP 0xE350 (free play draw function)

get_credit_or_freeplay (0xE392)
; This loads the A register with either the amount of credits or
; '0' when in freeplay. This has the effect of going back to the
; attract screen instead of the start game screen when you get a
; game over in free play mode.
AD 17 40  ; LDA dipsw2_in
29 10     ; AND 0x10 (check DIP 5 is on)
D0 06     ; BNE 6
AD 30 05  ; LDA #0x0530 (load credit amount into A)
4C 9B A8  ; JMP 0xA89B
A9 30     ; LDA #0x30 (load 'zero credits' into A)
4C 9B A8  ; JMP 0xA89B

Update to new function (0xA027):
20 85 E3 ; JSR draw_freeplay_or_credit

Update to new free-play check (0xA898):
4C 92 E3 ; JMP get_credit_or_freeplay