snake6502/src/program.asm

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NASM
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; ENTRY OF PROGRAM
; ----------------------------------------------------------------------
start:
; Clear screen, initialize keyboard, restore interrupts
jsr $ff81
; Disable all interrupts
sei
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; Turn off CIA interrupts and (possibly) flush the pending queue
ldy #$7f
sty $dc0d
sty $dd0d
lda $dc0d
lda $dd0d
; Set Interrupt Request Mask as we want IRQ by raster beam
lda #$1
sta $d01a
; Store in $314 address of our custom interrupt handler
ldx #<irq ; least significant byte
ldy #>irq ; most significant byte
stx $314
sty $315
; Set raster beam to trigger interrupt at row zero
lda #$00
sta $d012
; Bit#0 of $d011 is used as bit#9 of $d012, and must be zero
lda $d011
and #$7f
sta $d011
; Initialize player for first song
lda #0
jsr sidtune
; Initialize MultiColor mode
jsr multicolorInit
; Zero-fill zeropage variables
lda #$0
ldx #$90
zeroFillZeroPage:
dex
sta $0,x
cpx #$2
bne zeroFillZeroPage
; Set status as first-time intro playing
lda #ST_INTRO0
sta status
; Enable interrupts
cli
; Reset screen (and other parameters) to play intro
jsr introreset
intro0running: ; Cycle here until SPACE or `Q` is pressed
jsr $ffe4 ; GETIN
cmp #$20 ; Is it SPACE?
beq intro0end ; if yes, go to intro0end and start game (see)
cmp #$51 ; Is it Q?
bne intro0running ; If not, keep looping here,
jmp $fce2 ; else, reset the computer
; Intro is finished, now it's time to start the proper game
intro0end:
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; Set current level pointer to list start
lda #<levelsList
sta levelPointer
lda #>levelsList
sta levelPointer + 1
; clear score
lda #$00
sta score
sta score + 1
; Set init variables of the level
jsr levelresetvar
; Set status as level select
; (then it will enter in status play)
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lda #ST_LEVEL_TITLE
sta status
endless:
; Loop waiting for gameover
lda status
cmp #ST_END ; is status equal to end ?
bne endless ; if not, just wait looping here, else...
jsr introreset ; reset variables for intro
lda #ST_INTRO0
sta status ; put machine into play intro status
jmp intro0running ; and go there waiting for keypress
; Interrupt Handler
; ----------------------------------------------------------------------
irq:
; Things that must be done every interrupt (50Hz)
; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Acknoweledge IRQ
dec $d019
; Save registers in stack
pha
txa
pha
tya
pha
#if DEBUG = 1
; Change background to visually see the ISR timing
lda #2
sta $d020
#endif
; Check status and call appropriate sub-routine
; Sort of switch-case
lda status
cmp #ST_INTRO0
bne checkStatusIntro1
jsr statusIntro0
jmp checkEndStatus
checkStatusIntro1:
cmp #ST_INTRO1
bne checkStatusPlay
jsr statusIntro1
jmp checkEndStatus
checkStatusPlay:
cmp #ST_PLAY
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bne checkStatusDelay
jsr statusPlay
jmp checkEndStatus
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checkStatusDelay:
cmp #ST_DELAY
bne checkStatusLevelTitle
jsr statusDelay
jmp checkEndStatus
checkStatusLevelTitle:
cmp #ST_LEVEL_TITLE
bne checkStatusLevelLoad
jsr statusLevelTitle
jmp checkEndStatus
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checkStatusLevelLoad:
cmp #ST_LEVEL_LOAD
bne checkEndStatus
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jsr statusLevelLoad
jmp checkEndStatus
checkEndStatus:
#if DEBUG = 1
; Change background to show how much time does music take for each interrupt
lda #1
sta $d020
#endif
; Play music
jsr sidtune + 3
jsr sidtune + 3
jsr sidtune + 3
jsr sidtune + 3
jsr sidtune + 3
; Increase random value
inc random
#if DEBUG = 1
; Change background back again to visally see ISR timing
lda #11
sta $d020
#endif
; Restore registers from stack
pla
tay
pla
tax
pla
; Go to original system routine
jmp $ea31