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Unsigned Longword Multiplication on the M68k
*-----------------------------------------------------------
* Title : Unsigned Longword Multiplication.
* Written by : Stuart Haidon
* Date : 2014-06-22
*-----------------------------------------------------------
* Longword Unsigned Multiplication.
* Pre:
* +4 : Multiplicand.
* +0 : Multiplier.
* Post:
* +4 : Result Low.
* +0 : Result High.
long_mulu:
move.l a6,-(a7) * A6 = Argument pointer.
move.l a7,a6
add.l #8,a6
movem.l d0-d6,-(a7)
move.l 4(a6),d0 * D0 = Multiplicand.
move.l 0(a6),d1 * D1 = Multiplier.
moveq.l #0,d2
moveq.l #0,d3
moveq.l #0,d4
moveq.l #0,d5
moveq.l #0,d6
* Compute z0.
move.w d0,d2
mulu d1,d2 * D2 = z0.
* Compute first term of z1.
* Multiplicand.
move.w d0,d3
swap d0
move.w d0,d5
add.l d5,d3 * D3 = Multiplicand of first term of z1.
* Multiplier.
move.w d1,d4
swap d1
move.w d1,d5
add.l d5,d4 * D4 = Multiplier of first term of z1.
* Multiply the two.
if.l d3 <gt> #$FFFF OR.l d4 <gt> #$FFFF then.s
move.l d3,-(a7)
move.l d4,-(a7)
jsr long_mulu
move.l (a7)+,d6 * D6 = Result high.
move.l (a7)+,d3 * D3 = Result low.
else.s
mulu d4,d3
endi * D3 = First term of z1.
* Subtract z0.
sub.l d2,d3 * D3 = First and third terms of z1.
if.l <cs> then.s
sub.l #1,d6
endi
* Compute z2.
moveq #0,d4
move.w d0,d4
mulu d1,d4 * D4 = z2.
* Complete z1.
sub.l d4,d3 * D3 = z1.
if.l <cs> then.s
sub.l #1,d6
endi
moveq #0,d0
moveq #0,d1
* Calculate result.
move.l d4,d0 * D0 = High.
move.l d2,d1 * D1 = Low.
* Add high to lower word of D0, low to higher word of D1.
swap d3
move.w d3,d5
add.l d5,d0
move.w #0,d3
add.l d3,d1
if.l <cs> then.s
add.l #1,d0
endi
* Add D6 << 16 to D0. Can just swap as higher word is always 0.
swap d6
add.l d6,d0
move.l d0,0(a6)
move.l d1,4(a6)
movem.l (a7)+,d0-d6
move.l (a7)+,a6
rts
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