;Joystick Assembly Language Module for C64

JYSTKS  EQU $02     ;Number of Joysticks

;Joystick Bit Masks
JOYUP   EQU $01     ;Bit 0 - Up    
JOYDN   EQU $02     ;Bit 1 - Down  
JOYLF   EQU $04     ;Bit 2 - Left  
JOYRT   EQU $08     ;Bit 3 - Right 
JOYB0   EQU $10     ;Bit 4 - Button

;Read Joystick
JOYSTK: CMP #JYSTKS ;If Invalid Joystick#
        BCS JOYSTZ  ;  Return Error
        EOR #$01    ;Invert Joystick Number
        TAX         ;and Copy to X Register
        LDA $DC00,X ;Read Joystick
        EOR #$FF    ;Invert and
        AND #$1F    ;Mask Bits
        RTS
JOYSTZ: LDA #$FF    ;Return Error
        RTS