round.s

linux-2.4.29操作系统的源码

	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