4cade/src/hw.joystick.a

;------------------------------------------------------------------------------
; HasJoystick
; Checks whether computer has joystick connected
;
; in:    none
; out:   C set if joystick found
;        C clear if no joystick found
;        other flags clobbered
;        A,X clobbered
;        Y preserved
;
; adapted from "Prince of Persia" by Jordan Mechner
; (c) 1989 Broderbund Software
; https://github.com/jmechner/Prince-of-Persia-Apple-II/blob/master/01%20POP%20Source/Source/GRAFIX.S#L1225
;------------------------------------------------------------------------------
HasJoystick
         lda   #0
         sta   @joyX
         sta   @joyY
         lda   $C070                 ; reset analog input timers
@loop    ldx   #1
@1       lda   $C064,x               ; check timer input
         bpl   @beat
         inc   @joyX,x               ; still high, increment counter
@nextpdl dex
         bpl   @1
         lda   $C064
         ora   $C065
         bpl   @yes                  ; both inputs low, we're done
         lda   @joyX
         ora   @joyY
         bpl   @loop                 ; do it again
@no      clc
         +HIDE_NEXT_BYTE
@yes     sec
         rts
@beat    nop
         bpl   @nextpdl              ; kill time
@joyX    !byte 0
@joyY    !byte 0