561 lines
12 KiB
NASM
561 lines
12 KiB
NASM
processor 6502
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; Zero page utilities
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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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tileMem = $a0
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printStatusString = $a3
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org $1000
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; SID tune (previously properly cleaned, see HVSC)
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; ----------------------------------------------------------------------
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. = $1000
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sidtune:
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INCBIN "snake.sid"
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; Data section
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; ----------------------------------------------------------------------
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. = $2100
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; Number of interrupt
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; Used as counter to be decremented to do some things less frequently
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irqn:
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BYTE
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; Direction of the snake (2,4,6,8 as down,left,right,up)
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; 5 means `pause`
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direction:
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BYTE
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; Snake head coordinates in video memory
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snakeX:
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BYTE
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snakeY:
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BYTE
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; Parameters for calcTileMem
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calcTileX:
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BYTE
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calcTileY:
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BYTE
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; List start and length
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listStart:
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BYTE
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length:
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BYTE
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; Random value
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random:
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BYTE
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; Costants
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; ----------------------------------------------------------------------
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; Screen width and height
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screenW:
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BYTE #40
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screenH:
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BYTE #24
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snakeTile:
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BYTE #81
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snakeColor:
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BYTE #5
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foodTile:
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BYTE #90
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foodColor:
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BYTE #15
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gameoverString:
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BYTE "GAME IS OVER"
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BYTE #0
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; List
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; ----------------------------------------------------------------------
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. = $2200
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listX:
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. = $2300
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listY:
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; ENTRY OF PROGRAM
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; ----------------------------------------------------------------------
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. = $2400
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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
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ldx #39
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upperbarLoop:
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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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; Turn off CIA interrupts and (eventually) flush the pending queue
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ldy #$7f
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sty $dc0d
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sty $dd0d
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lda $dc0d
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lda $dd0d
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; Set Interrupt Request Mask as we want IRQ by raster beam
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lda #$1
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sta $d01a
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; Store in $314 address of our custom interrupt handler
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ldx #<irq ; least significant byte
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ldy #>irq ; most significant byte
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stx $314
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sty $315
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; Set raster beam to trigger interrupt at row zero
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lda #$00
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sta $d012
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; Bit#0 of $d011 is used as bit#9 of $d012, and must be zero
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lda $d011
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and #$7f
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sta $d011
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; Initialize player for first song
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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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; Enable interrupts
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cli
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jsr fullreset
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; Endless loop, show that there is enogh time after the interrupt
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endless:
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ldx $400
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inx
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stx $400
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jmp endless
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brk
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; Full reset
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; ----------------------------------------------------------------------
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fullreset:
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lda #4
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sta irqn
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lda #6
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sta direction
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lda #19
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sta snakeX
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lda #12
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sta snakeY
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lda #5
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sta listStart
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lda #5
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sta length
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rts
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; Interrupt Handler
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; ----------------------------------------------------------------------
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irq:
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; Acknoweledge IRQ
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dec $d019
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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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irqsometime:
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; Things that must be done only one in four interrupts (12 Hz)
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; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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; If I am here, counter reached 0, so first reset it
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ldx #4
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stx irqn
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; Get pressed key and decide snake direction
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jsr $ffe4 ; Kernal routine GETIN
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cmp #0
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beq keybEndCheck ; if no key pressed, just skip this switch-case
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ldx #$7f ; else, if key pressed, reset random variable
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stx random
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keybCheckA: ; check for press of key `A`
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cmp #$41
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bne keybCheckS ; if not pressed `A`, just skip to next key to check
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lda direction ; else check if current direction is right
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cmp #6
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beq keybEndCheck ; if yes, you can't turn over yourself, so just skip to next key check
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lda #4
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sta direction ; else set direction to left and store new value
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jmp keybEndCheck ; skip all other key tests
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keybCheckS: ; simply re-do for S, D, W and other keys...
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cmp #$53
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bne keybCheckD
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lda direction
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cmp #8
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beq keybEndCheck
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lda #2
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sta direction
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jmp keybEndCheck
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keybCheckD:
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cmp #$44
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bne keybCheckW
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lda direction
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cmp #4
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beq keybEndCheck
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lda #6
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sta direction
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jmp keybEndCheck
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keybCheckW:
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cmp #$57
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bne keybCheckP
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lda direction
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cmp #2
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beq keybEndCheck
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lda #8
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sta direction
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jmp keybEndCheck
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keybCheckP:
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cmp #$50
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bne keybEndCheck
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lda #5
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sta direction
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jmp keybEndCheck
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keybEndCheck:
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; Get direction and move head accordingly
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lda direction
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dirCheck2: ; check if direction is down...
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cmp #2
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bne dirCheck4 ; if not down, then skip and check next direction,
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ldy snakeY ; else, direction is down, so get snakeY
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iny ; increment snakeY (keep in mind that screen up/down coordinates are reversed)
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sty snakeY ; update snakeY
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dirCheck4: ; simply re-do for other directions...
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cmp #4
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bne dirCheck6
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ldx snakeX
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dex
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stx snakeX
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dirCheck6:
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cmp #6
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bne dirCheck8
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ldx snakeX
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inx
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stx snakeX
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dirCheck8:
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cmp #8
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bne dirCheck5
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ldy snakeY
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dey
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sty snakeY
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dirCheck5:
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cmp #5
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bne dirEndCheck
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jmp skipPauseTests
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dirEndCheck:
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; Check screen boundaries overflow
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lda snakeX
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cmp #40
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bne overCheckX0 ; if snakeX is not 40, then all ok, skip to next test
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lda #0 ; else, there is an overflow, so trespass screen border
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sta snakeX
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overCheckX0: ; simply re-do for every side of the screen
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lda snakeX
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cmp #$ff
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bne overCheckY1
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lda #39
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sta snakeX
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overCheckY1:
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lda snakeY
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cmp #25
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bne overCheckY0
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lda #1
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sta snakeY
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overCheckY0
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lda snakeY
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cmp #0
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bne overEndCheck
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lda #24
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sta snakeY
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overEndCheck:
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; Put new head coordinates in list
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ldy listStart
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lda snakeX
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sta listX,y
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lda snakeY
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sta listY,y
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iny
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sty listStart
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ldy #0
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; Check for food eat / wall hit / self-eat
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; - - - - - - - - - - - - - - - - - - - - - -
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; --- Food eat ---
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lda snakeX ; calc head location in memory
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sta calcTileX
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lda snakeY
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sta calcTileY
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jsr calcTileMem
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lda (tileMem),y ; read content of head memory location
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cmp foodTile
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bne checkSelfEat ; if memory does not contain food, then skip to self-eat test...
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ldx length ; else, increment snake length,
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inx
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stx length
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genFood:
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ldx random ; increment random value
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inx
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stx random
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txa
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sta calcTileX ; use it as tile X-coordinate
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lsr ; divide it by 8; maximum expected is 32 (256/8)
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lsr
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lsr
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cmp #18 ; test if it is still > 18
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bcc foodNoMod ; value must be < 18,
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; because it is the highest value v
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; for which v * 40 + 256 < 1000
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; this ensures that piece if food will be placed
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; within visible screen
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sec ; if value is > 18, then subtract 18; now it
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sbc #18 ; surely is less than 18
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foodNoMod:
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bne foodNoLow ; if it is 0, then set 1 (avoid first line)
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lda #1
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foodNoLow:
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sta calcTileY ; use this new value as tile Y-coordinate
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jsr calcTileMem ; calc its address in memory
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lda (tileMem),y ; check if memory is empty
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cmp #$20 ; is there a space?
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bne genFood ; if not, must generate another number
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lda foodTile ; else, just put that fucking piece of food there
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sta (tileMem),y
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lda #$d4
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clc
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adc tileMem + 1
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sta tileMem + 1
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lda foodColor
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sta (tileMem),y
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jsr printScore ; print score
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jmp checkEndSelfEat
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checkEndFood:
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; --- Self eat ---
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checkSelfEat:
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cmp snakeTile
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bne checkEndSelfEat
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jmp gameover
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checkEndSelfEat:
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; Draw snake head
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lda snakeX ; calc char address in video memory, and put snakeTile
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sta calcTileX
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lda snakeY
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sta calcTileY
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jsr calcTileMem
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lda snakeTile
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sta (tileMem),y
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lda #$d4 ; add #$d400 to previous address (obtain color memory
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clc ; correspondent), and put snakeColor
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adc tileMem + 1
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sta tileMem + 1
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lda snakeColor
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sta (tileMem),y
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; Erase snake tail
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lda listStart ; take start of list, and subtract snake length,
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sec ; to obtain index of end of list
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sbc length
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tax ; use previous value as index in list, and calc video memory address
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lda listX,x
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sta calcTileX
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lda listY,x
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sta calcTileY
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jsr calcTileMem
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lda #$20 ; just put a space to erase snake tail tile
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sta (tileMem),y
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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 $ea31
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brk
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; Game is over
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; ----------------------------------------------------------------------
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gameover:
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lda #<gameoverString
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sta printStatusString
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lda #>gameoverString
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sta printStatusString + 1
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jsr printStatus
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jsr sidtune
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sei
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ldx #$ff
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ldy #$ff
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gameoverLoop:
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nop
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nop
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nop
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nop
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dex
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bne gameoverLoop
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dey
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bne gameoverLoop
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jmp 64738
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; Subroutines
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; ----------------------------------------------------------------------
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; Do some math to calculate tile address in video memory
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; using x,y coordinates
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; Formula: addr = $400 + y * screenW + x
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calcTileMem:
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; Save registers
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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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; Set tileMem to $400
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lda #$00
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sta tileMem
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lda #$04
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sta tileMem + 1
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ldy calcTileY ; Get head Y coordinate
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calcTileMult:
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tya
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beq calcTileEnd ; if Y is equal to zero, nothing to do, just skip moltiplication, else...
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dey ; decrement Y
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clc
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lda screenW ; A = screen width = 40
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adc tileMem ; A = screen width + tileMem (low)
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sta tileMem ; update tileMem (low)
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lda #0 ; do the same with higher byte: A = 0
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adc tileMem + 1 ; add (eventual) carry
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sta tileMem + 1 ; update tileMem (high)
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jmp calcTileMult ; do again until Y == 0
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calcTileEnd: ; now multiplication is ended, so add X
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lda calcTileX
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adc tileMem
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sta tileMem
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lda #0
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adc tileMem + 1
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sta tileMem + 1
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; Restore old registers
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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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rts
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; Print score
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printScore:
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; Push registers on stack (save)
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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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; Take length in A and copy also in X
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lda length
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tax
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lsr ; Take most significant nibble
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lsr
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lsr
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lsr
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jsr printDigit
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sta $410 ; print msb char at $410
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txa ; Take least significant nibble (use previous copy)
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and #$0f
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jsr printDigit
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sta $411 ; print lsb char at $411
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; Pull registers from stack (restore)
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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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rts
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; Transform a base-36 digit into a Commodore screen code
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; Leave input digit in accumulator; returns output screen code in accumulator
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printDigit:
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cmp #10
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bcs printDigitL ; if it is not a decimal digit, then go to printDigitL
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clc ; it is a decimal digit! Just add `0` (48)
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adc #48
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rts
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printDigitL: ; it is not a decimal digit, then...
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sec
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sbc #10 ; take away 10
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clc
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adc #1 ; add 1, so you obtain something in [A-F]
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rts
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; Print null-terminated string on status bar
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; address of string is given in input using memory location printStatusString
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printStatus:
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ldy #0
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printStatusLoop:
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lda (printStatusString),y
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beq printStatusEnd
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cmp #$20
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bne printStatusSkipSpace
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lda #$60
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printStatusSkipSpace:
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sec
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sbc #$40 ; convert from standard ASCII to Commodore screen code
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sta $413,y
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iny
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jmp printStatusLoop
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printStatusEnd:
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rts
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