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|
; memory layout {{{
; rom {{{
.ROMBANKMAP
BANKSTOTAL 2
BANKSIZE $4000
BANKS 1
BANKSIZE $2000
BANKS 1
.ENDRO
; }}}
; ram {{{
.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
.ENUM $0000
buttons_pressed DB
sleeping DB
game_state DB ; 0: menu, 1: playing, 2: redrawing
head_x DB
head_y DB
direction DB ; 0: up, 1: down, 2: left, 3: right
frame_skip DB
frame_count DB
.ENDE
; }}}
; }}}
; other configuration {{{
.asciitable
; just use ascii for now
.enda
; }}}
; prg {{{
.bank 0
.org $0000
; main codepath {{{
; initialization {{{
; 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
; First wait for vblank to make sure PPU is ready
- BIT $2002 ; bit 7 of $2002 is reset once vblank ends
BPL - ; 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
STA game_state
STA direction
STA frame_count
LDA #$80
STA head_x
STA head_y
LDA #30
STA frame_skip
; Second wait for vblank, PPU is ready after this
- BIT $2002
BPL -
; 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 #%10000000 ; enable NMI interrupts
STA $2000
JSR end_game
; }}}
; main loop {{{
loop:
INC sleeping
- LDA sleeping
BNE -
JSR read_controller1
LDA game_state
BNE +
JSR start_screen_loop
JMP loop
+ JSR game_loop
JMP loop
; }}}
; }}}
; nmi interrupt {{{
NMI:
PHA
TXA
PHA
TYA
PHA
LDA game_state
CMP #$01
BNE end_nmi
LDA head_y
STA $0200
LDA #$00
STA $0201
LDA #$00
STA $0202
LDA head_x
STA $0203
LDA #$00
STA $2003
LDA #$02
STA $4014
end_nmi:
LDA #$00
STA sleeping
PLA
TAY
PLA
TAX
PLA
RTI
; }}}
; subroutines {{{
start_screen_loop: ; {{{
handle_start:
LDA buttons_pressed
AND #%00010000
CMP #$00
BEQ end_start_screen_loop
JSR start_game
end_start_screen_loop:
RTS ; }}}
game_loop: ; {{{
handle_up:
LDA buttons_pressed
AND #%00001000
CMP #$00
BEQ handle_down
LDA #$00
STA direction
handle_down:
LDA buttons_pressed
AND #%00000100
CMP #$00
BEQ handle_left
LDA #$01
STA direction
handle_left:
LDA buttons_pressed
AND #%00000010
CMP #$00
BEQ handle_right
LDA #$02
STA direction
handle_right:
LDA buttons_pressed
AND #%00000001
CMP #$00
BEQ handle_frame
LDA #$03
STA direction
handle_frame:
LDX frame_count
INX
STX frame_count
CPX frame_skip
BMI end_game_loop
LDA #$00
STA frame_count
set_offset:
LDX #$F8 ; i.e., -8
LDA direction
AND #%00000001 ; low bit determines negative or positive
BEQ set_axis
LDX #$08
set_axis:
LDY #$01
LDA direction
AND #%00000010 ; high bit determines which axis to change
BEQ apply_direction
LDY #$00
apply_direction:
TXA
CLC
ADC head_x, y ; head_x offset by 1 is head_y
STA head_x, y
check_collisions
LDA head_x
CMP #$18
BCC collision
CMP #$E8
BCS collision
LDA head_y
CMP #$18
BCC collision
CMP #$E8
BCS collision
JMP end_game_loop
collision:
JSR end_game
end_game_loop:
RTS ; }}}
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 ; }}}
start_game: ; {{{
LDA #$01
STA game_state
RTS ; }}}
end_game: ; {{{
LDA #$02
STA game_state
- BIT $2002
BPL -
LDA #%00000000
STA $2001 ; disable rendering (since this will take longer than vblank)
LDA #$20
STA $2006
LDA #$00
STA $2006
LDA #$20
LDX #$0F
-- LDY #$00
- STA $2007
INY
CPY #$20
BNE -
DEX
BNE --
LDY #$00
- LDA intro_screen, y
STA $2007
INY
CPY #$20
BNE -
LDA #$20
LDX #$0E
-- LDY #$00
- STA $2007
INY
CPY #$20
BNE -
DEX
BNE --
LDA #%00011000
STA $2001 ; reenable rendering
LDA #$00
STA game_state
RTS ; }}}
; }}}
; data {{{
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
; }}}
intro_screen: ; {{{
.asc " SNAKE "
; }}}
; }}}
.orga $FFFA ;first of the three vectors starts here
; interrupt vectors {{{
.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
; }}}
; }}}
; chr {{{
.bank 1 slot 1
.org $0000
.incbin "sprites.chr"
; }}}
|