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_DIVDF3.S
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1993-07-29
|
3KB
|
110 lines
| double floating point divide 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 1.2, kub 01-90 :
| added support for denormalized numbers
|
| Revision 1.1, kub 12-89 :
| Ported over to 68k assembler
|
| Revision 1.0:
| original 8088 code from P.S.Housel
BIAS8 = 0x3FF-1
.text
.even
.globl __divdf3, ___divdf3
.globl __infinitydf
__divdf3:
___divdf3:
lea sp@(4),a0 | pointer to parameters u and v
moveml d2-d7,sp@- | save registers
moveml a0@,d4-d5/d6-d7 | d4-d5 = u, d6-d7 = v
movel d4,d0 | d0 = u.exp
swap d0
movew d0,d2 | d2 = u.sign
lsrw #4,d0
andw #0x07ff,d0 | kill sign bit
movel d6,d1 | d1 = v.exp
swap d1
eorw d1,d2 | d2 = u.sign ^ v.sign (in bit 31)
lsrw #4,d1
andw #0x07ff,d1 | kill sign bit
andl #0x0fffff,d4 | remove exponent from u.mantissa
tstw d0 | check for zero exponent - no leading "1"
beq 0f
orl #0x100000,d4 | restore implied leading "1"
bra 1f
0: addw #1,d0 | "normalize" exponent
1: movel d4,d3
orl d5,d3
beq retz | dividing zero
andl #0x0fffff,d6 | remove exponent from v.mantissa
tstw d1 | check for zero exponent - no leading "1"
beq 0f
orl #0x100000,d6 | restore implied leading "1"
bra 1f
0: addw #1,d1 | "normalize" exponent
1: movel d6,d3
orl d7,d3
beq divz | divide by zero
movew d2,a0 | save sign
subw d1,d0 | subtract exponents,
addw #BIAS8-11+1,d0 | add bias back in, account for shift
addw #66,d0 | add loop offset, +2 for extra rounding bits
| for denormalized numbers (2 implied by dbra)
movew #24,d1 | bit number for "implied" pos (+4 for rounding)
movel #-1,d2 | zero the quotient
movel #-1,d3 | (for speed it is a one's complement)
subl d7,d5 | initial subtraction,
subxl d6,d4 | u = u - v
2:
btst d1,d2 | divide until 1 in implied position
beq 5f
addl d5,d5
addxl d4,d4
bcs 4f | if carry is set, add, else subtract
addxl d3,d3 | shift quotient and set bit zero
addxl d2,d2
subl d7,d5 | subtract
subxl d6,d4 | u = u - v
dbra d0,2b | give up if result is denormalized
bra 5f
4:
addxl d3,d3 | shift quotient and clear bit zero
addxl d2,d2
addl d7,d5 | add (restore)
addxl d6,d4 | u = u + v
dbra d0,2b | give up if result is denormalized
5: subw #2,d0 | remove rounding offset for denormalized nums
notl d2 | invert quotient to get it right
notl d3
movel d2,d4 | save quotient mantissa
movel d3,d5
movew a0,d2 | get sign back
clrw d1 | zero rounding bits
jmp norm_df | (registers on stack removed by norm_df)
retz: clrl d0 | zero destination
clrl d1
moveml sp@+,d2-d7
rts | no normalization needed
divz: moveml __infinitydf,d0-d1 | return infinty value
moveml sp@+,d2-d7 | should really cause trap ?!?
rts