/ tst_and_clr.s

/ EMACS_MODES: !c !fill


/ This C-callable routine performs an atomic test-and-clear operation
/ on a single character variable whose address is passed it as parameter
/ pvar.  It clears the variable and returns 1 if its original value
/ was 1, 0 otherwise.  It uses the rorb instruction to guarantee atomicity.
/
/ Calling sequence:
/	tst_and_clr (pvar)
/	char	*pvar;
/ Returns:
/	R0 == 0 if *pvar was 0
/	R0 == 1 if *pvar was 1
/ Destroys R1
/

	.globl	_tst_and_clr

pvar	=	2			/ ptr. to var to be tested

_tst_and_clr:
	mov	pvar(sp),r1		/ point to var
	clr	r0			/ assume 0 return (note that this also
					/ clears the carry bit, for the rotate)
	rorb	(r1)			/ do it
	jcc	1f			/ if var was 0, branch
	inc	r0			/ it was 1, return 1
1:
	rts	pc