game.asm

; line of basic to call my program
; 10 SYS (2304)
*=$0801
        BYTE    $0E, $08, $0A, $00, $9E, $20, $28, $32
        BYTE    $33, $30, $34, $29, $00, $00, $00


; start my program at $0900
; BASIC programs memory starts at $0800,
; so I'll use that for my variables and stuff
*=$0900

; player related addresses (system)
PLAYER_X = $d000 ; sprite position x
PLAYER_Y = $d001 ; sprite position y
PLAYER_INPUT = $dc01 ; joystick 1 input
; player related addresses (basic ram)
PLAYER_SPEED_X = $0810
PLAYER_SPEED_Y = $0811

; routine to clear the screen
CLEARSCREEN
        lda #$02 ; colour we want to clear to
        sta $d020 ; border colour address
        sta $d021 ; screen colour address
        lda #$20 ; character to put into memory (space)
        ldx #$00 ; start x at 0 for loopyloops
_loop
        sta $0400,x ; 1st quarter of screen
        sta $0500,x ; 2nd quarter of screen
        sta $0600,x ; 3rd quarter of screen
        sta $0700,x ; 4th quarter of screen
        inx
        bne _loop ; loop until x is 0 again

SPRITEINIT
        ; point sprite 1 to $2000, where we load it
        lda #$80 ; 0x80 * 64 bytes (1 sprite) = $2000
        sta $07f8 ; address of sprite 1 pointer

        ; enable sprite 1
        lda #%00000001
        sta $d015 ; address of enabled sprites byte

        ; set default position
        lda #$80
        sta PLAYER_X 
        sta PLAYER_Y 

; main loopyloop
GAMELOOP
        jsr PROCESS_INPUT
        jmp GAMELOOP

; routine for moving the player around
PROCESS_INPUT
_checkup
        ; check if up is pressed on joystick
        lda PLAYER_INPUT
        and #%00000001
        bne _checkdown
        ; up was pressed!
        ; to control the speed of the moving, I increment a 'speed' variable
        ; each frame the button is pressed, and only when that variable
        ; overflows, we move the sprite
        ; so this is essentially moving at 1 pixel every 256 frames??!?!
        inc PLAYER_SPEED_Y
        bne _checkdown
        ; we can move up one pixel :D
        dec PLAYER_Y
_checkdown
        ; check if down is pressed on joystick
        lda PLAYER_INPUT
        and #%00000010
        bne _checkleft
        ; down was pressed!
        inc PLAYER_SPEED_Y
        bne _checkleft
        ; we can move down
        inc PLAYER_Y
_checkleft
        lda PLAYER_INPUT
        and #%00000100
        bne _checkright
        ; left was pressed!
        inc PLAYER_SPEED_X
        bne _checkright
        ; we can move left
        dec PLAYER_X
_checkright
        lda PLAYER_INPUT
        and #%00001000
        bne _inputend
        ; right was pressed!
        inc PLAYER_SPEED_X
        bne _inputend
        inc PLAYER_X
_inputend
        rts

; load my sprite into $2000 so we know where it is :D
*=$2000
        incbin "sprite1.bin"