Conscious use of assembler pre-processor directives, to avoid use of magic numbers and memory locations for costants
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e177fdcd86
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3ce810a482
67
snake.asm
67
snake.asm
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@ -15,13 +15,6 @@ 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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; Status of the game (costants pre-processor defined)
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ST_INTRO0 = 0
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ST_INTRO1 = 1
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ST_PLAY = 2
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ST_OUTRO = 3
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ST_END = 4
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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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@ -89,20 +82,24 @@ outroDelay
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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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; Status of the game (costants pre-processor defined, sort of enum)
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ST_INTRO0 = 0
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ST_INTRO1 = 1
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ST_PLAY = 2
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ST_OUTRO = 3
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ST_END = 4
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; Screen features
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SCREEN_W = 40
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SCREEN_H = 24
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; Snake features
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SNAKE_TILE = 81
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SNAKE_COLOR = 5
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; Food features
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FOOD_TILE = 90
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FOOD_COLOR = 15
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; Strings
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gameoverString:
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BYTE "GAME IS OVER"
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BYTE #0
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@ -240,7 +237,7 @@ upperbarLoop:
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bne upperbarLoop
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; Init game variables
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lda foodTile
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lda #FOOD_TILE
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sta $500 ; Put first piece of food
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lda #4
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sta irqn ; Initialize interrupt divider
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@ -595,7 +592,7 @@ overEndCheck:
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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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cmp #FOOD_TILE
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beq foodEaten ; if memory does contain food, then perform foodEaten actions,
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jmp checkSelfEat ; else just loooong jump to test if I ate myself
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foodEaten:
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@ -608,15 +605,15 @@ genFood:
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stx random
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txa
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genFoodX: ; calculate `random` modulo `screenW`
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genFoodX: ; calculate `random` modulo SCREEN_W
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sec
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sbc screenW
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cmp screenW
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sbc #SCREEN_W
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cmp #SCREEN_W
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bcs genFoodX
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sta calcTileX
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txa
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genFoodY: ; calculate `random` modulo 22 (22 = screenH - 1)
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genFoodY: ; calculate `random` modulo 22 (22 = SCREEN_H - 1)
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sec
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sbc #22
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cmp #22
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@ -648,14 +645,14 @@ foodOK:
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lda (tileMem),y ; check if memory is empty
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cmp #$20 ; is there a space?
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bne genFood ; if not, must generate another number
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lda foodTile ; else, just put that fucking piece of food there
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lda #FOOD_TILE ; else, just put that fucking piece of food there
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sta (tileMem),y
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lda #$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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lda #FOOD_COLOR
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sta (tileMem),y
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; print score at $10th column
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@ -668,26 +665,26 @@ checkEndFood:
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; --- Self eat ---
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checkSelfEat:
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cmp snakeTile
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cmp #SNAKE_TILE
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bne checkEndSelfEat
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jmp gameover
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checkEndSelfEat:
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; Draw snake head
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ldy #0
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lda snakeX ; calc char address in video memory, and put snakeTile
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lda snakeX ; calc char address in video memory, and put SNAKE_TILE
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sta calcTileX
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lda snakeY
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sta calcTileY
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jsr calcTileMem
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lda snakeTile
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lda #SNAKE_TILE
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sta (tileMem),y
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lda #$d4 ; add #$d400 to previous address (obtain color memory
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clc ; correspondent), and put snakeColor
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clc ; correspondent), and put SNAKE_COLOR
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adc tileMem + 1
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sta tileMem + 1
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lda snakeColor
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lda #SNAKE_COLOR
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sta (tileMem),y
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; Erase snake tail
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@ -743,7 +740,7 @@ statusOutroEnd:
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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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; Formula: addr = $400 + y * SCREEN_W + x
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calcTileMem:
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; Save registers
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pha
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@ -764,7 +761,7 @@ calcTileMult:
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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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lda #SCREEN_W ; 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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