Added a lot of useful comments to document this code for future reviews, for future me, and for other mads who still code for the 6502 inside a CBM64
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168
snake.asm
168
snake.asm
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@ -107,7 +107,7 @@ intro2string:
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intro3string:
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BYTE "(C) 2018"
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BYTE #0
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colorshade:
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colorshade: ; a shade from dark-gray to bright yellow, and vice-versa (40 columns)
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BYTE #11,#11,#11,#11,#11,#12,#12,#12,#12,#12,#5,#5,#5
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BYTE #13,#13,#13,#13,#7,#7,#7,#7,#7,#7
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BYTE #13,#13,#13,#13,#5,#5,#5,#12,#12,#12,#12,#12,#11,#11,#11,#11,#11
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@ -167,16 +167,18 @@ start:
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; Enable interrupts
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cli
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; Reset screen (and other parameters) to play intro
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jsr introreset
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intro0running:
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jsr $ffe4
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cmp #$20
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beq intro0end
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cmp #$51
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bne intro0running
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jmp $fce2
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intro0running: ; Cycle here until SPACE or `Q` is pressed
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jsr $ffe4 ; GETIN
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cmp #$20 ; Is it SPACE?
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beq intro0end ; if yes, go to intro0end and start game (see)
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cmp #$51 ; Is it Q?
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bne intro0running ; If not, keep looping here,
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jmp $fce2 ; else, reset the computer
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; Intro is finished, now it's time to start the proper game
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intro0end:
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; Set init variables of the game
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jsr fullreset
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@ -185,19 +187,15 @@ intro0end:
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sta status
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endless:
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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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; Loop waiting for gameover
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lda status
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cmp #3
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bne endless
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cmp #3 ; is status equal to 3 (gameover) ?
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bne endless ; if not, just wait looping here, else...
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jsr introreset
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jsr introreset ; reset variables for intro
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lda #0
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sta status
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jmp intro0running
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sta status ; put machine into 0 status (play intro)
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jmp intro0running ; and go there waiting for keypress
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; Full game reset
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; ----------------------------------------------------------------------
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@ -234,17 +232,17 @@ upperbarLoop:
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lda foodTile
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sta $500 ; Put first piece of food
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lda #4
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sta irqn
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sta irqn ; Initialize interrupt divider
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lda #6
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sta direction
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sta direction ; Snake must go right
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lda #19
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sta snakeX
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sta snakeX ; Snake is at screen center width...
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lda #12
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sta snakeY
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sta snakeY ; ... and height
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lda #5
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sta listStart
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sta listStart ; Beginning of snake tiles list
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lda #5
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sta length
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sta length ; Length of the list
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rts
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@ -263,11 +261,14 @@ introresetCLS:
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cpx #$ff
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bne introresetCLS
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; Copy shade colors from costant table to color RAM for 2nd and 4th
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; line of text. Soon there will be some text bouncing on these
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; lines
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ldx #39
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introresetColorShade
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lda colorshade,x
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sta $d828,x
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sta $d878,x
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sta $d828,x ; 2nd line
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sta $d878,x ; 4th line
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dex
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cpx #$ff
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bne introresetColorShade
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@ -278,19 +279,21 @@ introresetColorShade
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lda #0
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sta $d021 ; center
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lda #<intro2string
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sta printIntroString
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lda #>intro2string
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sta printIntroString + 1
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lda #$26
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sta introScreenStart
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lda #$07
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sta introScreenStart + 1
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jsr printIntro
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; Print website
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lda #<intro2string ; lsb of string address
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sta printIntroString ; put into lsb of source pointer
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lda #>intro2string ; do the same for msb of string address
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sta printIntroString + 1 ; put into msb of source pointer
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lda #$26 ; this is lsb of address of 20th line
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sta introScreenStart ; put into lsb of dest pointer
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lda #$07 ; do the same for msb of adress of 20th line
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sta introScreenStart + 1 ; put into msb of dest pointer
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jsr printIntro ; print
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lda #<intro3string
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sta printIntroString
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lda #>intro3string
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; Print Copyright
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lda #<intro3string ; the assembly is the same as above,
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sta printIntroString ; just change string to be printed
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lda #>intro3string ; and line (21th line)
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sta printIntroString + 1
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lda #$58
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sta introScreenStart
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@ -316,6 +319,11 @@ irq:
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pha
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; Check status
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; Sort of switch-case
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; 0 intro running
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; 1 for future use
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; 2 actual game running
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; 3 gameover
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lda status
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cmp #0
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bne checkStatus1
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@ -351,64 +359,83 @@ checkEndStatus:
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; Go to original system routine
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jmp $ea31
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; Currently status0 is the same as status1
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; status1 has just been reserved for future use
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status0:
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status1:
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; Decrement interrupt divider for the intro
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ldx introCounter
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dex
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stx introCounter
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cpx #0
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beq status1do
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rts
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beq status1do ; if divider is 0, then do status1do ...
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rts ; ... else just do nothing and return
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status1do:
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; Reset introCounter
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ldx #5
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stx introCounter
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; I want to print strings at different columns to make them
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; bounce across the screen, so take last introX and add introXinc,
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; then print string at that point. If introX is too far right, then
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; set introXinc as #$ff (equals -1) so next time introX will be
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; decremented by 1. And then, if introX is too far left, then
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; set introXinc as #$01 so next time will be moved to right again.
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lda introX
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clc
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adc introXinc
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adc introXinc ; this is #$01 or #$0ff, so actually it is +1 or -1
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sta introX
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cmp #19
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beq status1setSX
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cmp #0
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beq status1setDX
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jmp status1okset
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cmp #19 ; am I too far right?
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beq status1setSX ; if yes, set SX (left)
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cmp #0 ; am I too far left?
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beq status1setDX ; if yes, set DX (right)
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jmp status1okset ; else do nothing (aka, next time re-use current
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; increment value)
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status1setDX:
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lda #$01
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lda #$01 ; set introXinc as +1
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sta introXinc
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jmp status1okset
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status1setSX:
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lda #$ff
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lda #$ff ; set introXinc as -1
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sta introXinc
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jmp status1okset
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status1okset:
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; Print "SNAKE BY GIOMBA" (see above for pointer details)
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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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; $0428 is 2nd line (previously filled with color shades by reset routine)
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lda #$28
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clc
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adc introX
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adc introX ; just add X, to make it look like it has moved
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sta introScreenStart
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lda #$04
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sta introScreenStart + 1
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jsr printIntro
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; Print "PRESS SPACE TO PLAY"
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lda #<intro1string
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sta printIntroString
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lda #>intro1string
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sta printIntroString + 1
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; $0478 is 4th line (previously filled with color shades by reset routine)
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; add #19, then sub introX will make it move to other way of 2nd line
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lda #$78
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clc
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adc #19
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adc #19 ; add #19
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sec
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sbc introX
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sbc introX ; sub introX
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sta introScreenStart
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lda #$04
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sta introScreenStart + 1
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jsr printIntro
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; Some considerations on speed:
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; yes, maybe I should have put the string chars once in screen text memory
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; and then move it left and right. Should re-think about this.
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; For now, just return.
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rts
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status2: ; do Game
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@ -619,9 +646,11 @@ foodOK:
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lda foodColor
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sta (tileMem),y
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; print score at $10th column
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ldy #$10
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lda length
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jsr printByte ; print score
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jsr printByte
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jmp checkEndSelfEat
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checkEndFood:
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@ -675,9 +704,14 @@ gameover:
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sta printStatusString + 1
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jsr printStatus
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; Set gameover status
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; this way, the loop out of this interrupt, will know that we
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; finished, and play the intro again
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lda #3
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sta status
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rts
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; TODO : must be added a delay to let the player see her/his final score
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; before clearing the screen and starting with the intro again
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; Subroutines
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; ----------------------------------------------------------------------
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@ -787,19 +821,35 @@ printStatusEnd:
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rts
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; Print string for intro
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; Input parameters:
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; printIntroString pointer to string to be printed (source)
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; introScreenStart pointer to text video memory on screen where to print (dest)
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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 #$40
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bcc printIntroEndCheck
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lda (printIntroString),y ; get char from string
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beq printIntroEnd ; if zero, then end (string must be null-terminated)
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cmp #$40 ; is char greater or equal to #$40 = #64 = `@' ?
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bcc printIntroEndCheck ; if not, it is less, thus it must be
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; a full stop, comma, colon or something
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; that actually has the same value in both
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; true ASCII and in PET screen codes
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; otherwise, it is greater than `@`, so must
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; subtract 64 because CBM and its encodings
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; are simply a big shit
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sec
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sbc #$40
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printIntroEndCheck:
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sta (introScreenStart),y
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iny
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sta (introScreenStart),y ; put screen code to screen
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iny ; next char in string
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jmp printIntroLoop
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printIntroEnd:
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rts
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;
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; coded during december 2017
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; by giomba -- giomba@glgprograms.it
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; this software is free software and is distributed
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; under the terms of GNU GPL v3 license
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;
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