Z80 Routines:Input:DetectKeyPress

This routines tells whether any keys at all are pressed. Returns the result in z flag.

;out: z flag for no key, nz flag for any key pressed (except On)
detectkeypress:
	xor a
	out (1),a	; any part of keyboard is detected
	nop
	nop
	in a,(1)
	inc a
	ret