*******************************************************************************

* Examples of various (inefficient) ways of determining if a
* particular bit of the status register is set using various bit
* operations.  Recall that the lower five bits are, from highest to
* lowest: X, N , Z, V, C.
*
* All functions (XS, NS, ZS, VS, CS) return through (d0.w) a non-zero
* value if and only if the given bit was set.
*******************************************************************************

START

	section	0

_main:	
*	Various tests cases.  Comment/uncomment to use, and add your own.

*	move.b	#$80,d2
*	jsr	NS

*	move.b	#$80,d2
*	lsl.b	#1,d2
*	jsr	XS

*	move.b	#$10,d2
*	asl.b	#4,d2
*	jsr	VS

*	move.b	#$80,d2
*	lsl.b	#1,d2
*	jsr	CS

	move.b	#$70,d2
	lsl.b	#1,d2
	jsr	CS

	andi.l	#$FFFF,d0	; clear upper word

	move.l	d0,d1		; print result as long
	move.l	#3,d0
	trap	#15

	move.b	#9,d0		; exit
	trap	#15

*******************************************************************************

ZS:	move	sr,d0
	andi.w	#$4,d0		; use a mask
	rts			;   so that (d0) is zero if and only if Z was
				;   not set

* helper function for NS
fill:	tst.w	d0		; if (d0.w) = 0, return (d0.w) = 0, else
	beq	filldone	;   return (d0.w) = $FFFF
	move.w	#-1,d0
filldone
	rts
NS:	move	sr,d0
	btst	#3,d0		; use btst
	jsr	ZS		;   (d0) is non-zero if and only if N was 0
	jsr	fill		;   so complement it
	not.w	d0
	rts

XS:	move	sr,d0
	lsl.b	#3,d0		; shift so that X bit becomes left-most of byte
	jsr	NS		;   and test if the result is negative
	rts

VS:	move	sr,d0
	btst	#1,d0		; use btst and then branch
	bne	VSdone
	clr.w	d0
VSdone	rts

CS:	move	sr,d0
	bset	#0,d0		; bset also has effect of btst
	bne	CSdone
	clr.w	d0
CSdone	rts

	end	START