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.MEMORYMAP
DEFAULTSLOT 0
SLOTSIZE $4000
SLOT 0 $C000
SLOTSIZE $2000
SLOT 1 $0000 ; location doesn't matter, CHR data isn't in main memory
.ENDME
.ROMBANKMAP
BANKSTOTAL 2
BANKSIZE $4000
BANKS 1
BANKSIZE $2000
BANKS 1
.ENDRO
.ENUM $0000
buttons_pressed DB
sleeping DB
.ENDE
.bank 0
.org $0000
; the ppu takes two frames to initialize, so we have some time to do whatever
; initialization of our own that we want to while we wait. we choose here to
; set up cpu flags in the first frame and clear out system ram in the second
; frame (clearing out ram isn't at all necessary, but we can't do anything
; useful at this point anyway, so we may as well in order to make things more
; predictable).
RESET:
SEI ; disable IRQs
CLD ; disable decimal mode
LDX #$40
STX $4017.w ; disable APU frame IRQ
LDX #$FF
TXS ; Set up stack (grows down from $FF to $00, at $0100-$01FF)
INX ; now X = 0
STX $2000.w ; disable NMI (we'll enable it later once the ppu is ready)
STX $2001.w ; disable rendering (same)
STX $4010.w ; disable DMC IRQs
vblankwait1: ; First wait for vblank to make sure PPU is ready
BIT $2002 ; bit 7 of $2002 is reset once vblank ends
BPL vblankwait1 ; and bit 7 is what is checked by BPL
; set everything in ram ($0000-$07FF) to $00, except for $0200-$02FF which
; is conventionally used to hold sprite attribute data. we set that range
; to $FE, since that value as a position moves the sprites offscreen, and
; when the sprites are offscreen, it doesn't matter which sprites are
; selected or what their attributes are
clrmem:
LDA #$00
STA $0000, x
STA $0100, x
STA $0300, x
STA $0400, x
STA $0500, x
STA $0600, x
STA $0700, x
LDA #$FE
STA $0200, x
INX
BNE clrmem
; initialize variables in ram
LDA #$00
STA buttons_pressed
STA sleeping
vblankwait2: ; Second wait for vblank, PPU is ready after this
BIT $2002
BPL vblankwait2
; now that the ppu is ready, we can start initializing it
LoadPalettes:
LDA $2002 ; read PPU status to reset the high/low latch
LDA #$3F
STA $2006 ; write the high byte of $3F00 address
LDA #$00
STA $2006 ; write the low byte of $3F00 address
LDX #$00
LoadPalettesLoop:
LDA palette.w, x ;load palette byte
STA $2007 ;write to PPU
INX ;set index to next byte
CPX #$20
BNE LoadPalettesLoop ;if x = $20, 32 bytes copied, all done
LDA #%00010000 ; enable sprites
STA $2001
LDA #%10000000 ; enable NMI interrupts
STA $2000
loop:
INC sleeping
sleep:
LDA sleeping
BNE sleep
JSR read_controller1
; game logic
JMP loop
NMI:
PHA
TXA
PHA
TYA
PHA
; drawing code
LDA #$00
STA sleeping
PLA
TAY
PLA
TAX
PLA
RTI
read_controller1:
; latch
LDA #$01
STA $4016
LDA #$00
STA $4016
; clock
LDX #$00
read_controller1_values:
CPX #$08
BPL end_read_controller1
LDA $4016
AND #%00000001
ASL buttons_pressed
ORA buttons_pressed
STA buttons_pressed
INX
JMP read_controller1_values
end_read_controller1:
RTS
palette:
.db $0F,$31,$32,$33,$0F,$35,$36,$37,$0F,$39,$3A,$3B,$0F,$3D,$3E,$0F
.db $0F,$1C,$15,$14,$0F,$02,$38,$3C,$0F,$1C,$15,$14,$0F,$02,$38,$3C
.orga $FFFA ;first of the three vectors starts here
.dw NMI ;when an NMI happens (once per frame if enabled) the
;processor will jump to the label NMI:
.dw RESET ;when the processor first turns on or is reset, it will jump
;to the label RESET:
.dw 0 ;external interrupt IRQ is not used in this tutorial
.bank 1 slot 1
.org $0000
.incbin "sprites.chr"
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