fast joystick support in assembler

The same but a bit faster:

  ;; first we check the keyboard:
  xor a
  call GTSTCK
  or a
  call z, checkJoystick  ;; if keyboard is not pressed, then check joystick
  ;; ... do whatever we want to do with the input
  ret

checkJoystick:
  inc a
  jp GTSTCK

An another approach is to directly read the PSG.

A link to the scanned Dutch article (the code is commented in English):
http://msx.hansotten.com/uploads/msxdocs/joyml.pdf

... or maybe this is more easy to understand:

assume 2 buttons unless you would love to support more.

Try this code as well. Fast and direct:

;Standard keyboard mapping/ this is the same for joystick
; bit	7	  6	  5		    4		    3		    2		  1		  0
;		     	Z 	C 		  X   	  right	  left	down	up	(keyboard)
;
Readjoystick:	

  di
	ld	a, 15	; Lee el puerto de joystick y almacena
	out	($A0), a	; los estados en las variables.
	in	a, ($A2)
	and	10101111b
	out	($A1), a
	ld	a, 14
	out	($A0), a
	ei
	in	a, ($A2) ;read left right up down and button 1 and 2

By direct access I use:

GetJoy1:
	ld	a,15
;	di
	out	(0a0h),a
	in	a,(0a2h)	; Read register 15
	and	08f		; and 10001111
	out	(0a1h),a	; Write to register 15

	ld	a,14
	out	(0a0h),a
;	ei
	in	a,(0a2h)	; Read register 14
	ret

GetJoy2:
	ld	a,15
;	di
	out	(0a0h),a
	in	a,(0a2h)	; Read register 15
	or	040h		; set bit 6
	out	(0a1h),a	; Write to register 15

	ld	a,14
	out	(0a0h),a
;	ei
	in	a,(0a2h)	; Read register 14
	ret

; EI/DI is needed depending your program