|
  |	round.sa 3.4 7/29/91
  |
  |	handle rounding and normalization tasks
  |
  |
  |
  |		Copyright (C) Motorola, Inc. 1990
  |			All Rights Reserved
  |
  |       For details on the license for this file, please see the
  |       file, README, in this same directory.
  
  |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).
  |
  
  	.global	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.
  |
  	bsrs	ext_grs			|this subroutine looks at the
  |					:rounding precision and sets
  |					;the appropriate g-r-s bits.
  	tstl	%d0			|if grs are zero, go force
  	bne	rnd_cont		|lower bits to zero for size
  
  	swap	%d1			|set up d1.w for round prec.
  	bra	truncate
  
  rnd_cont:
  |
  | Use rounding mode as an index into a jump table for these modes.
  |
  	orl	#inx2a_mask,USER_FPSR(%a6) |set inex2/ainex
  	lea	mode_tab,%a1
  	movel	(%a1,%d1.w*4),%a1
  	jmp	(%a1)
  |
  | Jump table indexed by rounding mode in d1.w.  All following assumes
  | grs != 0.
  |
  mode_tab:
  	.long	rnd_near
  	.long	rnd_zero
  	.long	rnd_mnus
  	.long	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.
  	tstb	LOCAL_SGN(%a0)		|check for sign
  	bmi	truncate		|if positive then truncate
  	movel	#0xffffffff,%d0		|force g,r,s to be all f's
  	lea	add_to_l,%a1
  	movel	(%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.
  	tstb	LOCAL_SGN(%a0)		|check for sign
  	bpl	truncate		|if negative then truncate
  	movel	#0xffffffff,%d0		|force g,r,s to be all f's
  	lea	add_to_l,%a1
  	movel	(%a1,%d1.w*4),%a1
  	jmp	(%a1)
  |
  |	ROUND ZERO
  |
  |	Always truncate.
  rnd_zero:
  	swap	%d1			|set up d1 for round prec.
  	bra	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.
  	asll	#1,%d0			|shift g-bit to c-bit
  	bcc	truncate		|if (g=1) then
  	lea	add_to_l,%a1
  	movel	(%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
  	cmpiw	#0,%d1
  	bnes	sgl_or_dbl
  	bras	end_ext_grs
  
  sgl_or_dbl:
  	moveml	%d2/%d3,-(%a7)		|make some temp registers
  	cmpiw	#1,%d1
  	bnes	grs_dbl
  grs_sgl:
  	bfextu	LOCAL_HI(%a0){#24:#2},%d3	|sgl prec. g-r are 2 bits right
  	movel	#30,%d2			|of the sgl prec. limits
  	lsll	%d2,%d3			|shift g-r bits to MSB of d3
  	movel	LOCAL_HI(%a0),%d2		|get word 2 for s-bit test
  	andil	#0x0000003f,%d2		|s bit is the or of all other
  	bnes	st_stky			|bits to the right of g-r
  	tstl	LOCAL_LO(%a0)		|test lower mantissa
  	bnes	st_stky			|if any are set, set sticky
  	tstl	%d0			|test original g,r,s
  	bnes	st_stky			|if any are set, set sticky
  	bras	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
  	movel	#30,%d2			|of the dbl prec. limits
  	lsll	%d2,%d3			|shift g-r bits to the MSB of d3
  	movel	LOCAL_LO(%a0),%d2		|get lower mantissa  for s-bit test
  	andil	#0x000001ff,%d2		|s bit is the or-ing of all
  	bnes	st_stky			|other bits to the right of g-r
  	tstl	%d0			|test word original g,r,s
  	bnes	st_stky			|if any are set, set sticky
  	bras	end_sd			|if clear, exit
  st_stky:
  	bset	#rnd_stky_bit,%d3
  end_sd:
  	movel	%d3,%d0			|return grs to d0
  	moveml	(%a7)+,%d2/%d3		|restore scratch registers
  end_ext_grs:
  	swap	%d1			|restore d1 to original
  	rts
  
  |*******************  Local Equates
  	.set	ad_1_sgl,0x00000100	|  constant to add 1 to l-bit in sgl prec
  	.set	ad_1_dbl,0x00000800	|  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:
  	.long	add_ext
  	.long	add_sgl
  	.long	add_dbl
  	.long	add_dbl
  |
  |	ADD SINGLE
  |
  add_sgl:
  	addl	#ad_1_sgl,LOCAL_HI(%a0)
  	bccs	scc_clr			|no mantissa overflow
  	roxrw  LOCAL_HI(%a0)		|shift v-bit back in
  	roxrw  LOCAL_HI+2(%a0)		|shift v-bit back in
  	addw	#0x1,LOCAL_EX(%a0)	|and incr exponent
  scc_clr:
  	tstl	%d0			|test for rs = 0
  	bnes	sgl_done
  	andiw  #0xfe00,LOCAL_HI+2(%a0)	|clear the l-bit
  sgl_done:
  	andil	#0xffffff00,LOCAL_HI(%a0) |truncate bits beyond sgl limit
  	clrl	LOCAL_LO(%a0)		|clear d2
  	rts
  
  |
  |	ADD EXTENDED
  |
  add_ext:
  	addql  #1,LOCAL_LO(%a0)		|add 1 to l-bit
  	bccs	xcc_clr			|test for carry out
  	addql  #1,LOCAL_HI(%a0)		|propagate carry
  	bccs	xcc_clr
  	roxrw  LOCAL_HI(%a0)		|mant is 0 so restore v-bit
  	roxrw  LOCAL_HI+2(%a0)		|mant is 0 so restore v-bit
  	roxrw	LOCAL_LO(%a0)
  	roxrw	LOCAL_LO+2(%a0)
  	addw	#0x1,LOCAL_EX(%a0)	|and inc exp
  xcc_clr:
  	tstl	%d0			|test rs = 0
  	bnes	add_ext_done
  	andib	#0xfe,LOCAL_LO+3(%a0)	|clear the l bit
  add_ext_done:
  	rts
  |
  |	ADD DOUBLE
  |
  add_dbl:
  	addl	#ad_1_dbl,LOCAL_LO(%a0)
  	bccs	dcc_clr
  	addql	#1,LOCAL_HI(%a0)		|propagate carry
  	bccs	dcc_clr
  	roxrw	LOCAL_HI(%a0)		|mant is 0 so restore v-bit
  	roxrw	LOCAL_HI+2(%a0)		|mant is 0 so restore v-bit
  	roxrw	LOCAL_LO(%a0)
  	roxrw	LOCAL_LO+2(%a0)
  	addw	#0x1,LOCAL_EX(%a0)	|incr exponent
  dcc_clr:
  	tstl	%d0			|test for rs = 0
  	bnes	dbl_done
  	andiw	#0xf000,LOCAL_LO+2(%a0)	|clear the l-bit
  
  dbl_done:
  	andil	#0xfffff800,LOCAL_LO(%a0) |truncate bits beyond dbl limit
  	rts
  
  error:
  	rts
  |
  | Truncate all other bits
  |
  trunct:
  	.long	end_rnd
  	.long	sgl_done
  	.long	dbl_done
  	.long	dbl_done
  
  truncate:
  	lea	trunct,%a1
  	movel	(%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
  |
  	.global	nrm_zero
  nrm_zero:
  	movew	LOCAL_EX(%a0),%d0
  	cmpw   #64,%d0          |see if exp > 64
  	bmis	d0_less
  	bsr	nrm_set		|exp > 64 so exp won't exceed 0
  	rts
  d0_less:
  	moveml	%d2/%d3/%d5/%d6,-(%a7)
  	movel	LOCAL_HI(%a0),%d1
  	movel	LOCAL_LO(%a0),%d2
  
  	bfffo	%d1{#0:#32},%d3	|get the distance to the first 1
  |				;in ms mant
  	beqs	ms_clr		|branch if no bits were set
  	cmpw	%d3,%d0		|of X>Y
  	bmis	greater		|then exp will go past 0 (neg) if
  |				;it is just shifted
  	bsr	nrm_set		|else exp won't go past 0
  	moveml	(%a7)+,%d2/%d3/%d5/%d6
  	rts
  greater:
  	movel	%d2,%d6		|save ls mant in d6
  	lsll	%d0,%d2		|shift ls mant by count
  	lsll	%d0,%d1		|shift ms mant by count
  	movel	#32,%d5
  	subl	%d0,%d5		|make op a denorm by shifting bits
  	lsrl	%d5,%d6		|by the number in the exp, then
  |				;set exp = 0.
  	orl	%d6,%d1		|shift the ls mant bits into the ms mant
  	movel	#0,%d0		|same as if decremented exp to 0
  |				;while shifting
  	movew	%d0,LOCAL_EX(%a0)
  	movel	%d1,LOCAL_HI(%a0)
  	movel	%d2,LOCAL_LO(%a0)
  	moveml	(%a7)+,%d2/%d3/%d5/%d6
  	rts
  ms_clr:
  	bfffo	%d2{#0:#32},%d3	|check if any bits set in ls mant
  	beqs	all_clr		|branch if none set
  	addw	#32,%d3
  	cmpw	%d3,%d0		|if X>Y
  	bmis	greater		|then branch
  	bsr	nrm_set		|else exp won't go past 0
  	moveml	(%a7)+,%d2/%d3/%d5/%d6
  	rts
  all_clr:
  	movew	#0,LOCAL_EX(%a0)	|no mantissa bits set. Set exp = 0.
  	moveml	(%a7)+,%d2/%d3/%d5/%d6
  	rts
  |
  |	NRM_SET
  |
  	.global	nrm_set
  nrm_set:
  	movel	%d7,-(%a7)
  	bfffo	LOCAL_HI(%a0){#0:#32},%d7 |find first 1 in ms mant to d7)
  	beqs	lower		|branch if ms mant is all 0's
  
  	movel	%d6,-(%a7)
  
  	subw	%d7,LOCAL_EX(%a0)	|sub exponent by count
  	movel	LOCAL_HI(%a0),%d0	|d0 has ms mant
  	movel	LOCAL_LO(%a0),%d1 |d1 has ls mant
  
  	lsll	%d7,%d0		|shift first 1 to j bit position
  	movel	%d1,%d6		|copy ls mant into d6
  	lsll	%d7,%d6		|shift ls mant by count
  	movel	%d6,LOCAL_LO(%a0)	|store ls mant into memory
  	moveql	#32,%d6
  	subl	%d7,%d6		|continue shift
  	lsrl	%d6,%d1		|shift off all bits but those that will
  |				;be shifted into ms mant
  	orl	%d1,%d0		|shift the ls mant bits into the ms mant
  	movel	%d0,LOCAL_HI(%a0)	|store ms mant into memory
  	moveml	(%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:
  	movew	LOCAL_EX(%a0),%d0	|d0 has exponent
  	movel	LOCAL_LO(%a0),%d1	|d1 has ls mant
  	subw	#32,%d0		|account for ms mant being all zeros
  	bfffo	%d1{#0:#32},%d7	|find first 1 in ls mant to d7)
  	subw	%d7,%d0		|subtract shift count from exp
  	lsll	%d7,%d1		|shift first 1 to integer bit in ms mant
  	movew	%d0,LOCAL_EX(%a0)	|store ms mant
  	movel	%d1,LOCAL_HI(%a0)	|store exp
  	clrl	LOCAL_LO(%a0)	|clear ls mant
  	movel	(%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.
  |
  
  	.global	denorm
  denorm:
  	btstb	#6,LOCAL_EX(%a0)	|check for exponents between $7fff-$4000
  	beqs	no_sgn_ext
  	bsetb	#7,LOCAL_EX(%a0)	|sign extend if it is so
  no_sgn_ext:
  
  	cmpib	#0,%d0		|if 0 then extended precision
  	bnes	not_ext		|else branch
  
  	clrl	%d1		|load d1 with ext threshold
  	clrl	%d0		|clear the sticky flag
  	bsr	dnrm_lp		|denormalize the number
  	tstb	%d1		|check for inex
  	beq	no_inex		|if clr, no inex
  	bras	dnrm_inex	|if set, set inex
  
  not_ext:
  	cmpil	#1,%d0		|if 1 then single precision
  	beqs	load_sgl	|else must be 2, double prec
  
  load_dbl:
  	movew	#dbl_thresh,%d1	|put copy of threshold in d1
  	movel	%d1,%d0		|copy d1 into d0
  	subw	LOCAL_EX(%a0),%d0	|diff = threshold - exp
  	cmpw	#67,%d0		|if diff > 67 (mant + grs bits)
  	bpls	chk_stky	|then branch (all bits would be
  |				; shifted off in denorm routine)
  	clrl	%d0		|else clear the sticky flag
  	bsr	dnrm_lp		|denormalize the number
  	tstb	%d1		|check flag
  	beqs	no_inex		|if clr, no inex
  	bras	dnrm_inex	|if set, set inex
  
  load_sgl:
  	movew	#sgl_thresh,%d1	|put copy of threshold in d1
  	movel	%d1,%d0		|copy d1 into d0
  	subw	LOCAL_EX(%a0),%d0	|diff = threshold - exp
  	cmpw	#67,%d0		|if diff > 67 (mant + grs bits)
  	bpls	chk_stky	|then branch (all bits would be
  |				; shifted off in denorm routine)
  	clrl	%d0		|else clear the sticky flag
  	bsr	dnrm_lp		|denormalize the number
  	tstb	%d1		|check flag
  	beqs	no_inex		|if clr, no inex
  	bras	dnrm_inex	|if set, set inex
  
  chk_stky:
  	tstl	LOCAL_HI(%a0)	|check for any bits set
  	bnes	set_stky
  	tstl	LOCAL_LO(%a0)	|check for any bits set
  	bnes	set_stky
  	bras	clr_mant
  set_stky:
  	orl	#inx2a_mask,USER_FPSR(%a6) |set inex2/ainex
  	movel	#0x20000000,%d0	|set sticky bit in return value
  clr_mant:
  	movew	%d1,LOCAL_EX(%a0)		|load exp with threshold
  	movel	#0,LOCAL_HI(%a0)	|set d1 = 0 (ms mantissa)
  	movel	#0,LOCAL_LO(%a0)		|set d2 = 0 (ms mantissa)
  	rts
  dnrm_inex:
  	orl	#inx2a_mask,USER_FPSR(%a6) |set inex2/ainex
  no_inex:
  	rts
  
  |
  |	dnrm_lp --- normalize exponent/mantissa to specified threshold
  |
  | 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.
  |
  	.global	dnrm_lp
  dnrm_lp:
  	movel	%d2,-(%sp)		|save d2 for temp use
  	btstb	#E3,E_BYTE(%a6)		|test for type E3 exception
  	beqs	not_E3			|not type E3 exception
  	bfextu	WBTEMP_GRS(%a6){#6:#3},%d2	|extract guard,round, sticky  bit
  	movel	#29,%d0
  	lsll	%d0,%d2			|shift g,r,s to their positions
  	movel	%d2,%d0
  not_E3:
  	movel	(%sp)+,%d2		|restore d2
  	movel	LOCAL_LO(%a0),FP_SCR2+LOCAL_LO(%a6)
  	movel	%d0,FP_SCR2+LOCAL_GRS(%a6)
  	movel	%d1,%d0			|copy the denorm threshold
  	subw	LOCAL_EX(%a0),%d1		|d1 = threshold - uns exponent
  	bles	no_lp			|d1 <= 0
  	cmpw	#32,%d1
  	blts	case_1			|0 = d1 < 32
  	cmpw	#64,%d1
  	blts	case_2			|32 <= d1 < 64
  	bra	case_3			|d1 >= 64
  |
  | No normalization necessary
  |
  no_lp:
  	clrb	%d1			|set no inex2 reported
  	movel	FP_SCR2+LOCAL_GRS(%a6),%d0	|restore original g,r,s
  	rts
  |
  | case (0<d1<32)
  |
  case_1:
  	movel	%d2,-(%sp)
  	movew	%d0,LOCAL_EX(%a0)		|exponent = denorm threshold
  	movel	#32,%d0
  	subw	%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
  	movel	%d2,LOCAL_HI(%a0)		|store new LOCAL_HI
  	movel	%d1,LOCAL_LO(%a0)		|store new LOCAL_LO
  	clrb	%d1
  	bftst	%d0{#2:#30}
  	beqs	c1nstky
  	bsetl	#rnd_stky_bit,%d0
  	st	%d1
  c1nstky:
  	movel	FP_SCR2+LOCAL_GRS(%a6),%d2	|restore original g,r,s
  	andil	#0xe0000000,%d2		|clear all but G,R,S
  	tstl	%d2			|test if original G,R,S are clear
  	beqs	grs_clear
  	orl	#0x20000000,%d0		|set sticky bit in d0
  grs_clear:
  	andil	#0xe0000000,%d0		|clear all but G,R,S
  	movel	(%sp)+,%d2
  	rts
  |
  | case (32<=d1<64)
  |
  case_2:
  	movel	%d2,-(%sp)
  	movew	%d0,LOCAL_EX(%a0)		|unsigned exponent = threshold
  	subw	#32,%d1			|d1 now between 0 and 32
  	movel	#32,%d0
  	subw	%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}
  	bnes	c2_sstky		|bra if sticky bit to be set
  	bftst	FP_SCR2+LOCAL_LO(%a6){%d0:#32}
  	bnes	c2_sstky		|bra if sticky bit to be set
  	movel	%d1,%d0
  	clrb	%d1
  	bras	end_c2
  c2_sstky:
  	movel	%d1,%d0
  	bsetl	#rnd_stky_bit,%d0
  	st	%d1
  end_c2:
  	clrl	LOCAL_HI(%a0)		|store LOCAL_HI = 0
  	movel	%d2,LOCAL_LO(%a0)		|store LOCAL_LO
  	movel	FP_SCR2+LOCAL_GRS(%a6),%d2	|restore original g,r,s
  	andil	#0xe0000000,%d2		|clear all but G,R,S
  	tstl	%d2			|test if original G,R,S are clear
  	beqs	clear_grs
  	orl	#0x20000000,%d0		|set sticky bit in d0
  clear_grs:
  	andil	#0xe0000000,%d0		|get rid of all but G,R,S
  	movel	(%sp)+,%d2
  	rts
  |
  | d1 >= 64 Force the exponent to be the denorm threshold with the
  | correct sign.
  |
  case_3:
  	movew	%d0,LOCAL_EX(%a0)
  	tstw	LOCAL_SGN(%a0)
  	bges	c3con
  c3neg:
  	orl	#0x80000000,LOCAL_EX(%a0)
  c3con:
  	cmpw	#64,%d1
  	beqs	sixty_four
  	cmpw	#65,%d1
  	beqs	sixty_five
  |
  | Shift value is out of range.  Set d1 for inex2 flag and
  | return a zero with the given threshold.
  |
  	clrl	LOCAL_HI(%a0)
  	clrl	LOCAL_LO(%a0)
  	movel	#0x20000000,%d0
  	st	%d1
  	rts
  
  sixty_four:
  	movel	LOCAL_HI(%a0),%d0
  	bfextu	%d0{#2:#30},%d1
  	andil	#0xc0000000,%d0
  	bras	c3com
  
  sixty_five:
  	movel	LOCAL_HI(%a0),%d0
  	bfextu	%d0{#1:#31},%d1
  	andil	#0x80000000,%d0
  	lsrl	#1,%d0			|shift high bit into R bit
  
  c3com:
  	tstl	%d1
  	bnes	c3ssticky
  	tstl	LOCAL_LO(%a0)
  	bnes	c3ssticky
  	tstb	FP_SCR2+LOCAL_GRS(%a6)
  	bnes	c3ssticky
  	clrb	%d1
  	bras	c3end
  
  c3ssticky:
  	bsetl	#rnd_stky_bit,%d0
  	st	%d1
  c3end:
  	clrl	LOCAL_HI(%a0)
  	clrl	LOCAL_LO(%a0)
  	rts
  
  	|end