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Talk:Programming:Integer Multiplication

1,085 bytes added, 23:42, 6 May 2007
New page: The FastMult routine using logs can be optimised somewhat from it's original by removing the slow SET instructions and extra direct addressing operation: <pre> FastMult: ld l,c l...
The FastMult routine using logs can be optimised somewhat from it's original by removing the slow SET instructions and extra direct addressing operation:

<pre>
FastMult:
ld l,c
ld h,&82
ld d,(hl) ; d = 32 * log_2(c)

ld l,b
ld a,(hl) ; a = 32 * log_2(b)

add a,d
ld l,a
ld a,0
adc a,0
ld h,a ; hl = d + a

add hl,hl
set 2,h ; hl = hl + $0400
set 7,h ; hl = hl + &8000

ld e,(hl)
inc hl
ld d,(hl) ; de = 2^((hl)/32)

ret
</pre>

to

<pre>
FastMult:
ld l,c
ld h,&82
ld d,(hl) ; d = 32 * log_2(c)

ld l,b
ld a,(hl) ; a = 32 * log_2(b)

add a,d
ld l,a
adc &42
sub l
ld h,a ; hl = d + a

add hl,hl

ld e,(hl)
inc l
ld d,(hl) ; de = 2^((hl)/32)

ret
</pre>

The add hl,hl can also be removed if the tables are split with low-byte in one page and high-byte in the other.
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