gistfile1.txt

MMC3 on the left, MMC5 on the right

mapper init ($FFF1):

PRG: makes $8000-9FFF swappable and $C000-DFFF fixed,
CHR: sets up 2x2kb lower and 4x1kb upper
(probably only PRG part matters here; this should probably be replaced with
 an entire MMC5 init routine somewhere else, which may also set up the CHR-ROM
 bank setup that is normally asserted when selecting banks on the MMC3,
 as well as PRG-RAM and other stuff)

lda #$00
sta $8000

new MMC5 init routine:

8kb PRG banks
1kb CHR banks
map last 16KB of PRG ROM to C000+
PRG ram present and write-enabled
nametable setting: horizontal mirroring
also try to imitate any details of MMC3 boot state (TODO)

lda #$03
sta $5100
sta $5101
lda #$FE (or #$BE when unexpanded)
sta $5116
lda #$FF (or #$BF when unexpanded)
sta $5116
lda #$02
sta $5102
lda #$01
sta $5103
lda #$50
sta $5105
lda #$00
sta $5113

IRQ disable (various):

sta $E000	lsr $5204

enable PRG RAM ($C04B, second to last bank?):
probably NOP this out, it'll be part of the new init

lda #$80
sta $A001

select $8000-9FFF ($D787):

PRG: select 8000-9FFF
CHR: sets up 4x1kb lower, 2x2kb upper

pha 		ora #$80
lda #$86	sta $5114
sta $38		sta $48
sta $8000	rts
pla
sta $48
sta $8001
rts

select $A000-BFFF ($D796):

PRG: select A000-BFFF
CHR: sets up 4x1kb lower, 2x2kb upper

pha 		ora #$80
lda #$87	sta $5115
sta $38		sta $49
sta $8000	rts
pla
sta $49
sta $8001
rts

select/execute bank 1D ($F804):

PRG: select 8000-9FFF temporarily to bank $1D, then execute $8000
CHR: sets up 4x1kb lower, 2x2kb upper

lda $48		lda $48
pha 		pha
ldx #$86	lda #$9D
stx $38 	sta $48
lda #$1D	sta $5114
sta $48		jsr $8000
stx $8000	pla
sta $8001	sta $48
jsr $8000	sta $5114
ldx #$86	rts
stx $38
pla
sta $48
stx $8000
sta $8001
rts

use banks $1D and value of $0600 ($F8A0):

PRG: select 8000-9FFF temporarily to bank $1D, then execute $8153
     select A000-BFFF temporarily to bank stored in RAM $0600
CHR: sets up 4x1kb lower, 2x2kb upper

initial routine could fit in here, maybe

lda $38		lda $48
pha 		pha
lda $48		lda #$9d
pha 		sta $5114
ldx #$86	lda $49
stx $38		pha
lda #$1d	lda $0600
sta $48		sta $49
stx $8000	sta $5115
sta $8001	jsr $8153
lda $49		pla
pha 		sta $49
ldx #$87	sta $5115
stx $38		pla
lda $0600	sta $48
sta $49		sta $5114
stx $8000	rts
sta $8001
jsr $8153
ldx #$87
stx $38
pla
sta $49
stx $8000
sta $8001
ldx #$86
stx $38
pla
sta $48
stx $8000
sta $8001
pla
sta $38
rts

refresh $8000 bank setting ($C397):
just NOP this out entirely, probably

lda $38
sta $8000

IRQ latch, reload, and enable ($CB66):

(note: what does the IRQ counter reload value indicate? scanline number?
this may need the entire subroutine it's part of to be reworked to calculate the
target scanline correctly)

9 bytes 	8 bytes

sta $C000	sta $5203
sta $C001	lda #$80
sta $E001	sta $5204

ingame CHR bank select ($C401):

CHR: $42 -> 2kb at $1000
     $43 -> 2kb at $1800
     $44 -> 1kb at $0000
     $45 -> 1kb at $0400
     $46 -> 1kb at $0800
     $47 -> 1kb at $0c00

idea: this routine could also repurpose an unused byte in the level data to be used as
the upper bits of the background CHR page numbers (could use four bytes, but it'd be a pain
to deal with this and the preset CHR bank combinations at the same time...)

42 bytes	35 bytes

lda $38		ldx #$03
pha 		-
ldx #$05	lda $44,x
-   		sta $5120,x
txa 		dex
ora #$80	bpl -
sta $38		lda $42
sta $8000	and #$FE
lda $42,x	sta $5124
sta $8001	ora #$01
dex 		sta $5125
bne -		lda $43
lda $01A5	and #$FE
bne +		sta $5126
lda #$80	ora #$01
sta $38		sta $5127
sta $8000	rts
lda $42		
sta $8001	
pla
sta $38
rts