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_ADDSUBS.S
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1993-07-29
|
3KB
|
122 lines
| single floating point add/subtract routine
|
| written by Kai-Uwe Bloem (I5110401@dbstu1.bitnet).
| Based on a 80x86 floating point packet from comp.os.minix, written by P.Housel
|
|
| Revision 1.3, kub 01-90 :
| added support for denormalized numbers
|
| Revision 1.2, kub 01-90 :
| replace far shifts by swaps to gain speed (more optimization is of course
| possible by doing shifts all in one intruction, but what about the rounding
| bits)
|
| Revision 1.1, kub 12-89 :
| Created single float version for 68000
|
| Revision 1.0:
| original 8088 code from P.S.Housel for double floats
.text
.even
.globl __subsf3, ___subsf3
.globl __addsf3, ___addsf3
__subsf3:
___subsf3:
eorb #0x80,sp@(8) | reverse sign of v
__addsf3:
___addsf3:
lea sp@(4),a0 | pointer to u and v parameter
moveml d2-d5,sp@- | save registers
moveml a0@,d4/d5 | d4 = v, d5 = u
movel d5,d0 | d0 = u.exp
swap d0
movel d5,d2 | d2.h = u.sign
movew d0,d2
lsrw #7,d0
andw #0xff,d0 | kill sign bit (exponent is 8 bits)
movel d4,d1 | d1 = v.exp
swap d1
eorw d1,d2 | d2.l = u.sign ^ v.sign
lsrw #7,d1
andw #0xff,d1 | kill sign bit (exponent is 8 bits)
andl #0x7fffff,d5 | remove exponent from mantissa
tstw d0 | check for zero exponent - no leading "1"
beq 0f
orl #0x800000,d5 | restore implied leading "1"
bra 1f
0: addw #1,d0 | "normalize" exponent
1:
andl #0x7fffff,d4 | remove exponent from mantissa
tstw d1 | check for zero exponent - no leading "1"
beq 0f
orl #0x800000,d4 | restore implied leading "1"
bra 1f
0: addw #1,d1 | "normalize" exponent
1:
clrw d3 | (put initial zero rounding bits in d3)
negw d1 | d1 = u.exp - v.exp
addw d0,d1
beq 5f | exponents are equal - no shifting neccessary
bgt 1f | not equal but no exchange neccessary
exg d4,d5 | exchange u and v
subw d1,d0 | d0 = u.exp - (u.exp - v.exp) = v.exp
negw d1
tstw d2 | d2.h = u.sign ^ (u.sign ^ v.sign) = v.sign
bpl 1f
bchg #31,d2
1:
cmpw #24,d1 | is u so much bigger that v is not
bge 7f | significant ?
movew #7-1,d3 | shift u left up to 7 bits to minimize loss
2:
addl d5,d5
subw #1,d0 | decrement exponent
subw #1,d1 | done shifting altogether ?
dbeq d3,2b | loop if still can shift u.mant more
clrw d3
cmpw #16,d1 | see if fast rotate possible
blt 4f
orb d4,d3 | set rounding bits
orb d2,d3
sne d2 | "sticky byte"
movew d4,d3
lsrw #8,d3
clrw d4 | rotate by swapping register halfs
swap d4
subw #16,d1
0:
lsrl #1,d4 | shift v.mant right the rest of the way
orb d3,d2 | set "sticky byte" if necessary
roxrw #1,d3 | shift into rounding bits
4: dbra d1,0b | loop
andb #1,d2 | see if "sticky bit" should be set
orb d2,d3
5:
tstw d2 | are the signs equal ?
bpl 6f | yes, no negate necessary
negb d3 | negate rounding bits and v.mant
negl d4
6:
addl d4,d5 | u.mant = u.mant + v.mant
bcs 7f | needn't negate
tstw d2 | opposite signs ?
bpl 7f | don't need to negate result
negb d3 | negate rounding bits and u.mant
negl d5
notl d2 | switch sign
7:
movel d5,d4 | move result for normalization
moveb d3,d1 | put rounding bits in d1 for norm_sf
swap d2 | put sign into d2 (exponent is in d0)
jmp norm_sf | leave registers on stack for norm_sf