;Joystick Assembly Language Module for C64

JYSTKS  EQU $04     ;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
        TAX         ;Copy Joystick # to X
        LDA $0284,X ;Read Trigger Control Register
        ROR         ;Move Bit 0 into Carry
        LDA $DC00,X ;Read Joystick Shadow Register
        EOR #$FF    ;Invert and
        AND #$0F    ;Mask Bits
        BCS JOYSTX  ;If Trigger Pressed
        ORA #$10    ;  Set Bit 4
HOYSTX: RTS
JOYSTZ: LDA #$FF    ;Return Error
        RTS