m68k/fpsp/round.sa

*	$NetBSD: round.sa,v 1.3 1994/10/26 07:49:24 cgd Exp $

*	MOTOROLA MICROPROCESSOR & MEMORY TECHNOLOGY GROUP
*	M68000 Hi-Performance Microprocessor Division
*	M68040 Software Package
*
*	M68040 Software Package Copyright (c) 1993, 1994 Motorola Inc.
*	All rights reserved.
*
*	THE SOFTWARE is provided on an "AS IS" basis and without warranty.
*	To the maximum extent permitted by applicable law,
*	MOTOROLA DISCLAIMS ALL WARRANTIES WHETHER EXPRESS OR IMPLIED,
*	INCLUDING IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
*	PARTICULAR PURPOSE and any warranty against infringement with
*	regard to the SOFTWARE (INCLUDING ANY MODIFIED VERSIONS THEREOF)
*	and any accompanying written materials.
*
*	To the maximum extent permitted by applicable law,
*	IN NO EVENT SHALL MOTOROLA BE LIABLE FOR ANY DAMAGES WHATSOEVER
*	(INCLUDING WITHOUT LIMITATION, DAMAGES FOR LOSS OF BUSINESS
*	PROFITS, BUSINESS INTERRUPTION, LOSS OF BUSINESS INFORMATION, OR
*	OTHER PECUNIARY LOSS) ARISING OF THE USE OR INABILITY TO USE THE
*	SOFTWARE.  Motorola assumes no responsibility for the maintenance
*	and support of the SOFTWARE.
*
*	You are hereby granted a copyright license to use, modify, and
*	distribute the SOFTWARE so long as this entire notice is retained
*	without alteration in any modified and/or redistributed versions,
*	and that such modified versions are clearly identified as such.
*	No licenses are granted by implication, estoppel or otherwise
*	under any patents or trademarks of Motorola, Inc.

*
*	round.sa 3.4 7/29/91
*
*	handle rounding and normalization tasks
*

ROUND	IDNT    2,1 Motorola 040 Floating Point Software Package

	section	8

	include	fpsp.h

*
*	round --- round result according to precision/mode
*
*	a0 points to the input operand in the internal extended format
*	d1(high word) contains rounding precision:
*		ext = $0000xxxx
*		sgl = $0001xxxx
*		dbl = $0002xxxx
*	d1(low word) contains rounding mode:
*		RN  = $xxxx0000
*		RZ  = $xxxx0001
*		RM  = $xxxx0010
*		RP  = $xxxx0011
*	d0{31:29} contains the g,r,s bits (extended)
*
*	On return the value pointed to by a0 is correctly rounded,
*	a0 is preserved and the g-r-s bits in d0 are cleared.
*	The result is not typed - the tag field is invalid.  The
*	result is still in the internal extended format.
*
*	The INEX bit of USER_FPSR will be set if the rounded result was
*	inexact (i.e. if any of the g-r-s bits were set).
*

	xdef	round
round:
* If g=r=s=0 then result is exact and round is done, else set
* the inex flag in status reg and continue.
*
	bsr.b	ext_grs			;this subroutine looks at the
*					:rounding precision and sets
*					;the appropriate g-r-s bits.
	tst.l	d0			;if grs are zero, go force
	bne.w	rnd_cont		;lower bits to zero for size

	swap	d1			;set up d1.w for round prec.
	bra.w	truncate

rnd_cont:
*
* Use rounding mode as an index into a jump table for these modes.
*
	or.l	#inx2a_mask,USER_FPSR(a6) ;set inex2/ainex
	lea	mode_tab,a1
	move.l	(a1,d1.w*4),a1
	jmp	(a1)
*
* Jump table indexed by rounding mode in d1.w.  All following assumes
* grs != 0.
*
mode_tab:
	dc.l	rnd_near
	dc.l	rnd_zero
	dc.l	rnd_mnus
	dc.l	rnd_plus
*
*	ROUND PLUS INFINITY
*
*	If sign of fp number = 0 (positive), then add 1 to l.
*
rnd_plus:
	swap 	d1			;set up d1 for round prec.
	tst.b	LOCAL_SGN(a0)		;check for sign
	bmi.w	truncate		;if positive then truncate
	move.l	#$ffffffff,d0		;force g,r,s to be all f's
	lea	add_to_l,a1
	move.l	(a1,d1.w*4),a1
	jmp	(a1)
*
*	ROUND MINUS INFINITY
*
*	If sign of fp number = 1 (negative), then add 1 to l.
*
rnd_mnus:
	swap 	d1			;set up d1 for round prec.
	tst.b	LOCAL_SGN(a0)		;check for sign
	bpl.w	truncate		;if negative then truncate
	move.l	#$ffffffff,d0		;force g,r,s to be all f's
	lea	add_to_l,a1
	move.l	(a1,d1.w*4),a1
	jmp	(a1)
*
*	ROUND ZERO
*
*	Always truncate.
rnd_zero:
	swap 	d1			;set up d1 for round prec.
	bra.w	truncate
*
*
*	ROUND NEAREST
*
*	If (g=1), then add 1 to l and if (r=s=0), then clear l
*	Note that this will round to even in case of a tie.
*
rnd_near:
	swap 	d1			;set up d1 for round prec.
	add.l	d0,d0			;shift g-bit to c-bit
	bcc.w	truncate		;if (g=1) then
	lea	add_to_l,a1
	move.l	(a1,d1.w*4),a1
	jmp	(a1)

*
*	ext_grs --- extract guard, round and sticky bits
*
* Input:	d1 =		PREC:ROUND
* Output:  	d0{31:29}=	guard, round, sticky
*
* The ext_grs extract the guard/round/sticky bits according to the
* selected rounding precision. It is called by the round subroutine
* only.  All registers except d0 are kept intact. d0 becomes an
* updated guard,round,sticky in d0{31:29}
*
* Notes: the ext_grs uses the round PREC, and therefore has to swap d1
*	 prior to usage, and needs to restore d1 to original.
*
ext_grs:
	swap	d1			;have d1.w point to round precision
	tst.w	d1
	bne.b	sgl_or_dbl
	bra.b	end_ext_grs

sgl_or_dbl:
	movem.l	d2/d3,-(a7)		;make some temp registers
	cmpi.w	#1,d1
	bne.b	grs_dbl
grs_sgl:
	bfextu	LOCAL_HI(a0){24:2},d3	;sgl prec. g-r are 2 bits right
	move.l	#30,d2			;of the sgl prec. limits
	lsl.l	d2,d3			;shift g-r bits to MSB of d3
	move.l	LOCAL_HI(a0),d2		;get word 2 for s-bit test
	andi.l	#$0000003f,d2		;s bit is the or of all other
	bne.b	st_stky			;bits to the right of g-r
	tst.l	LOCAL_LO(a0)		;test lower mantissa
	bne.b	st_stky			;if any are set, set sticky
	tst.l	d0			;test original g,r,s
	bne.b	st_stky			;if any are set, set sticky
	bra.b	end_sd			;if words 3 and 4 are clr, exit
grs_dbl:
	bfextu	LOCAL_LO(a0){21:2},d3	;dbl-prec. g-r are 2 bits right
	move.l	#30,d2			;of the dbl prec. limits
	lsl.l	d2,d3			;shift g-r bits to the MSB of d3
	move.l	LOCAL_LO(a0),d2		;get lower mantissa  for s-bit test
	andi.l	#$000001ff,d2		;s bit is the or-ing of all
	bne.b	st_stky			;other bits to the right of g-r
	tst.l	d0			;test word original g,r,s
	bne.b	st_stky			;if any are set, set sticky
	bra.b	end_sd			;if clear, exit
st_stky:
	bset	#rnd_stky_bit,d3
end_sd:
	move.l	d3,d0			;return grs to d0
	movem.l	(a7)+,d2/d3		;restore scratch registers
end_ext_grs:
	swap	d1			;restore d1 to original
	rts

********************  Local Equates
ad_1_sgl equ	$00000100	constant to add 1 to l-bit in sgl prec
ad_1_dbl equ	$00000800	constant to add 1 to l-bit in dbl prec


*Jump table for adding 1 to the l-bit indexed by rnd prec

add_to_l:
	dc.l	add_ext
	dc.l	add_sgl
	dc.l	add_dbl
	dc.l	add_dbl
*
*	ADD SINGLE
*
add_sgl:
	add.l	#ad_1_sgl,LOCAL_HI(a0)
	bcc.b	scc_clr			;no mantissa overflow
	roxr.w  LOCAL_HI(a0)		;shift v-bit back in
	roxr.w  LOCAL_HI+2(a0)		;shift v-bit back in
	add.w	#$1,LOCAL_EX(a0)	;and incr exponent
scc_clr:
	tst.l	d0			;test for rs = 0
	bne.b	sgl_done
	andi.w  #$fe00,LOCAL_HI+2(a0)	;clear the l-bit
sgl_done:
	andi.l	#$ffffff00,LOCAL_HI(a0) ;truncate bits beyond sgl limit
	clr.l	LOCAL_LO(a0)		;clear d2
	rts

*
*	ADD EXTENDED
*
add_ext:
	addq.l  #1,LOCAL_LO(a0)		;add 1 to l-bit
	bcc.b	xcc_clr			;test for carry out
	addq.l  #1,LOCAL_HI(a0)		;propogate carry
	bcc.b	xcc_clr
	roxr.w  LOCAL_HI(a0)		;mant is 0 so restore v-bit
	roxr.w  LOCAL_HI+2(a0)		;mant is 0 so restore v-bit
	roxr.w	LOCAL_LO(a0)
	roxr.w	LOCAL_LO+2(a0)
	add.w	#$1,LOCAL_EX(a0)	;and inc exp
xcc_clr:
	tst.l	d0			;test rs = 0
	bne.b	add_ext_done
	andi.b	#$fe,LOCAL_LO+3(a0)	;clear the l bit
add_ext_done:
	rts
*
*	ADD DOUBLE
*
add_dbl:
	add.l	#ad_1_dbl,LOCAL_LO(a0)
	bcc.b	dcc_clr
	addq.l	#1,LOCAL_HI(a0)		;propogate carry
	bcc.b	dcc_clr
	roxr.w	LOCAL_HI(a0)		;mant is 0 so restore v-bit
	roxr.w	LOCAL_HI+2(a0)		;mant is 0 so restore v-bit
	roxr.w	LOCAL_LO(a0)
	roxr.w	LOCAL_LO+2(a0)
	add.w	#$1,LOCAL_EX(a0)	;incr exponent
dcc_clr:
	tst.l	d0			;test for rs = 0
	bne.b	dbl_done
	andi.w	#$f000,LOCAL_LO+2(a0)	;clear the l-bit

dbl_done:
	andi.l	#$fffff800,LOCAL_LO(a0) ;truncate bits beyond dbl limit
	rts

error:
	rts
*
* Truncate all other bits
*
trunct:
	dc.l	end_rnd
	dc.l	sgl_done
	dc.l	dbl_done
	dc.l	dbl_done

truncate:
	lea	trunct,a1
	move.l	(a1,d1.w*4),a1
	jmp	(a1)

end_rnd:
	rts

*
*	NORMALIZE
*
* These routines (nrm_zero & nrm_set) normalize the unnorm.  This
* is done by shifting the mantissa left while decrementing the
* exponent.
*
* NRM_SET shifts and decrements until there is a 1 set in the integer
* bit of the mantissa (msb in d1).
*
* NRM_ZERO shifts and decrements until there is a 1 set in the integer
* bit of the mantissa (msb in d1) unless this would mean the exponent
* would go less than 0.  In that case the number becomes a denorm - the
* exponent (d0) is set to 0 and the mantissa (d1 & d2) is not
* normalized.
*
* Note that both routines have been optimized (for the worst case) and
* therefore do not have the easy to follow decrement/shift loop.
*
*	NRM_ZERO
*
*	Distance to first 1 bit in mantissa = X
*	Distance to 0 from exponent = Y
*	If X < Y
*	Then
*	  nrm_set
*	Else
*	  shift mantissa by Y
*	  set exponent = 0
*
*input:
*	FP_SCR1 = exponent, ms mantissa part, ls mantissa part
*output:
*	L_SCR1{4} = fpte15 or ete15 bit
*
	xdef	nrm_zero
nrm_zero:
	move.w	LOCAL_EX(a0),d0
	cmp.w   #64,d0          ;see if exp > 64
	bmi.b	d0_less
	bsr	nrm_set		;exp > 64 so exp won't exceed 0
	rts
d0_less:
	movem.l	d2/d3/d5/d6,-(a7)
	move.l	LOCAL_HI(a0),d1
	move.l	LOCAL_LO(a0),d2

	bfffo	d1{0:32},d3	;get the distance to the first 1
*				;in ms mant
	beq.b	ms_clr		;branch if no bits were set
	cmp.w	d3,d0		;of X>Y
	bmi.b	greater		;then exp will go past 0 (neg) if
*				;it is just shifted
	bsr	nrm_set		;else exp won't go past 0
	movem.l	(a7)+,d2/d3/d5/d6
	rts
greater:
	move.l	d2,d6		;save ls mant in d6
	lsl.l	d0,d2		;shift ls mant by count
	lsl.l	d0,d1		;shift ms mant by count
	move.l	#32,d5
	sub.l	d0,d5		;make op a denorm by shifting bits
	lsr.l	d5,d6		;by the number in the exp, then
*				;set exp = 0.
	or.l	d6,d1		;shift the ls mant bits into the ms mant
	clr.l	d0		;same as if decremented exp to 0
*				;while shifting
	move.w	d0,LOCAL_EX(a0)
	move.l	d1,LOCAL_HI(a0)
	move.l	d2,LOCAL_LO(a0)
	movem.l	(a7)+,d2/d3/d5/d6
	rts
ms_clr:
	bfffo	d2{0:32},d3	;check if any bits set in ls mant
	beq.b	all_clr		;branch if none set
	add.w	#32,d3
	cmp.w	d3,d0		;if X>Y
	bmi.b	greater		;then branch
	bsr	nrm_set		;else exp won't go past 0
	movem.l	(a7)+,d2/d3/d5/d6
	rts
all_clr:
	clr.w	LOCAL_EX(a0)	;no mantissa bits set. Set exp = 0.
	movem.l	(a7)+,d2/d3/d5/d6
	rts
*
*	NRM_SET
*
	xdef	nrm_set
nrm_set:
	move.l	d7,-(a7)
	bfffo	LOCAL_HI(a0){0:32},d7 ;find first 1 in ms mant to d7)
	beq.b	lower		;branch if ms mant is all 0's

	move.l	d6,-(a7)

	sub.w	d7,LOCAL_EX(a0)	;sub exponent by count
	move.l	LOCAL_HI(a0),d0	;d0 has ms mant
	move.l	LOCAL_LO(a0),d1 ;d1 has ls mant

	lsl.l	d7,d0		;shift first 1 to j bit position
	move.l	d1,d6		;copy ls mant into d6
	lsl.l	d7,d6		;shift ls mant by count
	move.l	d6,LOCAL_LO(a0)	;store ls mant into memory
	moveq.l	#32,d6
	sub.l	d7,d6		;continue shift
	lsr.l	d6,d1		;shift off all bits but those that will
*				;be shifted into ms mant
	or.l	d1,d0		;shift the ls mant bits into the ms mant
	move.l	d0,LOCAL_HI(a0)	;store ms mant into memory
	movem.l	(a7)+,d7/d6	;restore registers
	rts

*
* We get here if ms mant was = 0, and we assume ls mant has bits
* set (otherwise this would have been tagged a zero not a denorm).
*
lower:
	move.w	LOCAL_EX(a0),d0	;d0 has exponent
	move.l	LOCAL_LO(a0),d1	;d1 has ls mant
	sub.w	#32,d0		;account for ms mant being all zeros
	bfffo	d1{0:32},d7	;find first 1 in ls mant to d7)
	sub.w	d7,d0		;subtract shift count from exp
	lsl.l	d7,d1		;shift first 1 to integer bit in ms mant
	move.w	d0,LOCAL_EX(a0)	;store ms mant
	move.l	d1,LOCAL_HI(a0)	;store exp
	clr.l	LOCAL_LO(a0)	;clear ls mant
	move.l	(a7)+,d7
	rts
*
*	denorm --- denormalize an intermediate result
*
*	Used by underflow.
*
* Input:
*	a0	 points to the operand to be denormalized
*		 (in the internal extended format)
*
*	d0: 	 rounding precision
* Output:
*	a0	 points to the denormalized result
*		 (in the internal extended format)
*
*	d0 	is guard,round,sticky
*
* d0 comes into this routine with the rounding precision. It
* is then loaded with the denormalized exponent threshold for the
* rounding precision.
*

	xdef	denorm
denorm:
	btst.b	#6,LOCAL_EX(a0)	;check for exponents between $7fff-$4000
	beq.b	no_sgn_ext
	bset.b	#7,LOCAL_EX(a0)	;sign extend if it is so
no_sgn_ext:

	tst.b	d0		;if 0 then extended precision
	bne.b	not_ext		;else branch

	clr.l	d1		;load d1 with ext threshold
	clr.l	d0		;clear the sticky flag
	bsr	dnrm_lp		;denormalize the number
	tst.b	d1		;check for inex
	beq.w	no_inex		;if clr, no inex
	bra.b	dnrm_inex	;if set, set inex

not_ext:
	cmpi.l	#1,d0		;if 1 then single precision
	beq.b	load_sgl	;else must be 2, double prec

load_dbl:
	move.w	#dbl_thresh,d1	;put copy of threshold in d1
	move.l	d1,d0		;copy d1 into d0
	sub.w	LOCAL_EX(a0),d0	;diff = threshold - exp
	cmp.w	#67,d0		;if diff > 67 (mant + grs bits)
	bpl.b	chk_stky	;then branch (all bits would be
*				; shifted off in denorm routine)
	clr.l	d0		;else clear the sticky flag
	bsr	dnrm_lp		;denormalize the number
	tst.b	d1		;check flag
	beq.b	no_inex		;if clr, no inex
	bra.b	dnrm_inex	;if set, set inex

load_sgl:
	move.w	#sgl_thresh,d1	;put copy of threshold in d1
	move.l	d1,d0		;copy d1 into d0
	sub.w	LOCAL_EX(a0),d0	;diff = threshold - exp
	cmp.w	#67,d0		;if diff > 67 (mant + grs bits)
	bpl.b	chk_stky	;then branch (all bits would be
*				; shifted off in denorm routine)
	clr.l	d0		;else clear the sticky flag
	bsr	dnrm_lp		;denormalize the number
	tst.b	d1		;check flag
	beq.b	no_inex		;if clr, no inex
	bra.b	dnrm_inex	;if set, set inex

chk_stky:
	tst.l	LOCAL_HI(a0)	;check for any bits set
	bne.b	set_stky
	tst.l	LOCAL_LO(a0)	;check for any bits set
	bne.b	set_stky
	bra.b	clr_mant
set_stky:
	or.l	#inx2a_mask,USER_FPSR(a6) ;set inex2/ainex
	move.l	#$20000000,d0	;set sticky bit in return value
clr_mant:
	move.w	d1,LOCAL_EX(a0)		;load exp with threshold
	clr.l	LOCAL_HI(a0) 	;set d1 = 0 (ms mantissa)
	clr.l	LOCAL_LO(a0)		;set d2 = 0 (ms mantissa)
	rts
dnrm_inex:
	or.l	#inx2a_mask,USER_FPSR(a6) ;set inex2/ainex
no_inex:
	rts

*
*	dnrm_lp --- normalize exponent/mantissa to specified threshhold
*
* Input:
*	a0		points to the operand to be denormalized
*	d0{31:29} 	initial guard,round,sticky
*	d1{15:0}	denormalization threshold
* Output:
*	a0		points to the denormalized operand
*	d0{31:29}	final guard,round,sticky
*	d1.b		inexact flag:  all ones means inexact result
*
* The LOCAL_LO and LOCAL_GRS parts of the value are copied to FP_SCR2
* so that bfext can be used to extract the new low part of the mantissa.
* Dnrm_lp can be called with a0 pointing to ETEMP or WBTEMP and there
* is no LOCAL_GRS scratch word following it on the fsave frame.
*
	xdef	dnrm_lp
dnrm_lp:
	move.l	d2,-(sp)		;save d2 for temp use
	btst.b	#E3,E_BYTE(a6)		;test for type E3 exception
	beq.b	not_E3			;not type E3 exception
	bfextu	WBTEMP_GRS(a6){6:3},d2	;extract guard,round, sticky  bit
	move.l	#29,d0
	lsl.l	d0,d2			;shift g,r,s to their postions
	move.l	d2,d0
not_E3:
	move.l	(sp)+,d2		;restore d2
	move.l	LOCAL_LO(a0),FP_SCR2+LOCAL_LO(a6)
	move.l	d0,FP_SCR2+LOCAL_GRS(a6)
	move.l	d1,d0			;copy the denorm threshold
	sub.w	LOCAL_EX(a0),d1		;d1 = threshold - uns exponent
	ble.b	no_lp			;d1 <= 0
	cmp.w	#32,d1
	blt.b	case_1			;0 = d1 < 32
	cmp.w	#64,d1
	blt.b	case_2			;32 <= d1 < 64
	bra.w	case_3			;d1 >= 64
*
* No normalization necessary
*
no_lp:
	clr.b	d1			;set no inex2 reported
	move.l	FP_SCR2+LOCAL_GRS(a6),d0	;restore original g,r,s
	rts
*
* case (0<d1<32)
*
case_1:
	move.l	d2,-(sp)
	move.w	d0,LOCAL_EX(a0)		;exponent = denorm threshold
	move.l	#32,d0
	sub.w	d1,d0			;d0 = 32 - d1
	bfextu	LOCAL_EX(a0){d0:32},d2
	bfextu	d2{d1:d0},d2		;d2 = new LOCAL_HI
	bfextu	LOCAL_HI(a0){d0:32},d1	;d1 = new LOCAL_LO
	bfextu	FP_SCR2+LOCAL_LO(a6){d0:32},d0	;d0 = new G,R,S
	move.l	d2,LOCAL_HI(a0)		;store new LOCAL_HI
	move.l	d1,LOCAL_LO(a0)		;store new LOCAL_LO
	clr.b	d1
	bftst	d0{2:30}
	beq.b	c1nstky
	bset.l	#rnd_stky_bit,d0
	st.b	d1
c1nstky:
	move.l	FP_SCR2+LOCAL_GRS(a6),d2	;restore original g,r,s
	andi.l	#$e0000000,d2		;clear all but G,R,S
	tst.l	d2			;test if original G,R,S are clear
	beq.b	grs_clear
	or.l	#$20000000,d0		;set sticky bit in d0
grs_clear:
	andi.l	#$e0000000,d0		;clear all but G,R,S
	move.l	(sp)+,d2
	rts
*
* case (32<=d1<64)
*
case_2:
	move.l	d2,-(sp)
	move.w	d0,LOCAL_EX(a0)		;unsigned exponent = threshold
	sub.w	#32,d1			;d1 now between 0 and 32
	move.l	#32,d0
	sub.w	d1,d0			;d0 = 32 - d1
	bfextu	LOCAL_EX(a0){d0:32},d2
	bfextu	d2{d1:d0},d2		;d2 = new LOCAL_LO
	bfextu	LOCAL_HI(a0){d0:32},d1	;d1 = new G,R,S
	bftst	d1{2:30}
	bne.b	c2_sstky		;bra if sticky bit to be set
	bftst	FP_SCR2+LOCAL_LO(a6){d0:32}
	bne.b	c2_sstky		;bra if sticky bit to be set
	move.l	d1,d0
	clr.b	d1
	bra.b	end_c2
c2_sstky:
	move.l	d1,d0
	bset.l	#rnd_stky_bit,d0
	st.b	d1
end_c2:
	clr.l	LOCAL_HI(a0)		;store LOCAL_HI = 0
	move.l	d2,LOCAL_LO(a0)		;store LOCAL_LO
	move.l	FP_SCR2+LOCAL_GRS(a6),d2	;restore original g,r,s
	andi.l	#$e0000000,d2		;clear all but G,R,S
	tst.l	d2			;test if original G,R,S are clear
	beq.b	clear_grs
	or.l	#$20000000,d0		;set sticky bit in d0
clear_grs:
	andi.l	#$e0000000,d0		;get rid of all but G,R,S
	move.l	(sp)+,d2
	rts
*
* d1 >= 64 Force the exponent to be the denorm threshold with the
* correct sign.
*
case_3:
	move.w	d0,LOCAL_EX(a0)
	tst.w	LOCAL_SGN(a0)
	bge.b	c3con
c3neg:
	or.l	#$80000000,LOCAL_EX(a0)
c3con:
	cmp.w	#64,d1
	beq.b	sixty_four
	cmp.w	#65,d1
	beq.b	sixty_five
*
* Shift value is out of range.  Set d1 for inex2 flag and
* return a zero with the given threshold.
*
	clr.l	LOCAL_HI(a0)
	clr.l	LOCAL_LO(a0)
	move.l	#$20000000,d0
	st.b	d1
	rts

sixty_four:
	move.l	LOCAL_HI(a0),d0
	bfextu	d0{2:30},d1
	andi.l	#$c0000000,d0
	bra.b	c3com

sixty_five:
	move.l	LOCAL_HI(a0),d0
	bfextu	d0{1:31},d1
	andi.l	#$80000000,d0
	lsr.l	#1,d0			;shift high bit into R bit

c3com:
	tst.l	d1
	bne.b	c3ssticky
	tst.l	LOCAL_LO(a0)
	bne.b	c3ssticky
	tst.b	FP_SCR2+LOCAL_GRS(a6)
	bne.b	c3ssticky
	clr.b	d1
	bra.b	c3end

c3ssticky:
	bset.l	#rnd_stky_bit,d0
	st.b	d1
c3end:
	clr.l	LOCAL_HI(a0)
	clr.l	LOCAL_LO(a0)
	rts

	end