Reorganized code with statuses, to introduce an intro scene.
Added routine to print text during intro; things needs to be developed.
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snake.asm
203
snake.asm
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@ -4,10 +4,17 @@
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; ----------------------------------------------------------------------
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; Where is the snake head in video memory? Do math to calculate address
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; using $a0-$a1
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; using pointer at $a0-$a1
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tileMem = $a0
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; Pointer to status string ($a3-$a4)
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printStatusString = $a3
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; Pointer to intro string ($a3-$a4)
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printIntroString = $a3
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; Pointer to screen position where to print intro string ($fb-$fc)
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introScreenStart = $fb
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org $801
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. = $801 ; 10 SYS9216 ($2400) BASIC autostart
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BYTE #$0b,#$08,#$0a,#$00,#$9e,#$39,#$32,#$31,#$36,#$00,#$00,#$00
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@ -54,10 +61,14 @@ length:
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random:
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BYTE
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; Status (0 intro playing, 1 game running)
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; Status (0 fist time intro playing, 1 init (title) screen, 2 game running)
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status:
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BYTE
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; Intro counter
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introCounter:
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BYTE
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; Costants
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; ----------------------------------------------------------------------
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@ -78,6 +89,9 @@ foodColor:
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gameoverString:
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BYTE "GAME IS OVER"
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BYTE #0
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intro0string:
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BYTE "SNAKE BY GIOMBA"
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BYTE #0
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; List
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; ----------------------------------------------------------------------
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@ -94,23 +108,6 @@ start:
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; Clear screen, initialize keyboard, restore interrupts
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jsr $ff81
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; Set screen colors
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; - - - - - - - - -
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lda #12
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sta $d020 ; overscan
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lda #9
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sta $d021 ; center
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; Upper bar -- fill with reversed spaces, color yellow
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ldx #39
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upperbarLoop:
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lda #$a0 ; reversed color space
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sta $400,x
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lda #7
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sta $d800,x
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dex
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cpx #$ff
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bne upperbarLoop
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; Disable all interrupts
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sei
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@ -144,30 +141,26 @@ upperbarLoop:
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lda #0
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jsr sidtune
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; Put first piece of food
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lda foodTile
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sta $500
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; Set status as first-time intro playing
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lda #0
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sta status
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; Enable interrupts
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cli
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jsr fullreset
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; Simple intro
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intro:
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ldx 53280
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inx
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stx 53280
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intro0running:
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jsr $ffe4
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cmp #$20
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bne intro
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bne intro0running
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lda #1
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; Set init variables of the game
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jsr fullreset
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; Set status as game playing
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lda #2
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sta status
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endless:
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; Endless loop, show that there is enogh time after the interrupt
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; Endless loop, show that there is enough time after the interrupt
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ldx $400
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inx
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stx $400
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@ -176,6 +169,37 @@ endless:
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; Full reset
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; ----------------------------------------------------------------------
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fullreset:
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; Clear screen
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ldx #$ff
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lda #$20
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fullresetCLS:
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sta $400,x
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sta $500,x
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sta $600,x
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sta $700,x
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dex
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cpx #$ff
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bne fullresetCLS
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; Set screen colors
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lda #12
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sta $d020 ; overscan
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lda #9
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sta $d021 ; center
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; Upper bar -- fill with reversed spaces, color yellow
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ldx #39
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upperbarLoop:
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lda #$a0 ; reversed color space
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sta $400,x
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lda #7
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sta $d800,x
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dex
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cpx #$ff
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bne upperbarLoop
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; Init game variables
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lda foodTile
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sta $500 ; Put first piece of food
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lda #4
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sta irqn
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lda #6
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@ -188,29 +212,85 @@ fullreset:
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sta listStart
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lda #5
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sta length
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lda #0
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sta status
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rts
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; Interrupt Handler
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; ----------------------------------------------------------------------
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irq:
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; Things that must be done every interrupt (50Hz)
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; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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; Acknoweledge IRQ
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dec $d019
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; Save registers in stack
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pha
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txa
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pha
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tya
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pha
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; Check status
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lda status
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cmp #0
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bne checkStatus1
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jsr status0
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jmp checkEndStatus
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checkStatus1:
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cmp #1
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beq doGame ; if status is 1, must do game
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jmp irqalways ; else simply do every-interrupt tasks (always-do)
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bne checkStatus2
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jsr status1
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jmp checkEndStatus
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checkStatus2
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cmp #2
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bne checkEndStatus
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jsr status2
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jmp checkEndStatus
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checkEndStatus:
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doGame:
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; Play music
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jsr sidtune + 3
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; Increase random value
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ldx random
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inx
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stx random
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; Restore registers from stack
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pla
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tay
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pla
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tax
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pla
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; Go to original system routine
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jmp $ea31
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status0:
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status1:
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lda #<intro0string
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sta printIntroString
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lda #>intro0string
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sta printIntroString + 1
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lda #$00
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sta introScreenStart
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lda #$05
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sta introScreenStart + 1
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jsr printIntro
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ldx $401
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inx
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stx $401
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rts
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status2: ; do Game
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; Check counter
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ldx irqn
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dex
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stx irqn
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beq irqsometime ; if counter is 0, then do these "rare" things
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jmp irqalways ; else skip this section, and go to always-do interrupt routine
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beq irqsometime ; if counter is 0, then do these "rare" things
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rts ; else do nothing and simply return
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; as you can see, game actually runs at 12 Hz
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irqsometime:
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; Things that must be done only one in four interrupts (12 Hz)
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@ -396,14 +476,6 @@ foodOK:
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lda calcTileY
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jsr printByte
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; debug -- print snake head X,Y
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ldy #$1f
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lda snakeX
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jsr printByte
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ldy #$22
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lda snakeY
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jsr printByte
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ldy #0
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jsr calcTileMem ; calc food address in memory
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lda (tileMem),y ; check if memory is empty
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@ -464,22 +536,7 @@ checkEndSelfEat:
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skipPauseTests:
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irqalways:
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; Things that must be done every interrupt (50Hz)
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; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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; Play music
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jsr sidtune + 3
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; Increase random value
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ldx random
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inx
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stx random
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; Go to original system routine
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jmp $ea31
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brk
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rts
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; Game is over
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; ----------------------------------------------------------------------
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@ -611,3 +668,21 @@ printStatusSkipSpace:
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jmp printStatusLoop
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printStatusEnd:
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rts
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; Print string for intro
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printIntro:
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ldy #0
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printIntroLoop:
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lda (printIntroString),y
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beq printIntroEnd
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cmp #$20
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bne printIntroSkipSpace
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lda #$60
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printIntroSkipSpace:
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sec
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sbc #$40
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sta (introScreenStart),y
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iny
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jmp printIntroLoop
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printIntroEnd:
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rts
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