25 Jan
2008
25 Jan
'08
4:29 a.m.
dwight elvey wrote:
There is a trivial solution: a look up table.
add hl,hl ; double it
ld de,table ; base addr of table
add hl,de ; index
ld a,(hl) ; pick up quotient
...
table db 0,0 ; 0: quotient, remainder
db 0,1 ; 1
db 0,2
db 0,3
etc
From: korpela
at ssl.berkeley.edu
Based on an email I just had from the cpm news group, I thought
it might be fun to write some Z80 code to do the 0 to 799 as fast
as one could, with no size restriction. Just clocks count. Every time I see code like this for processors
without a native DIV, I
wonder if the same code ported to x86 would indeed outperform the native
DIV. Would it? I know that on a 286 or higher, where MUL and DIV were
greatly optimized to about 12 cycles, no; but what about on the original
808x, where MUL/DIV could take as much as 144 cycles?
In 286 and 386 you could
usually beat IDIV when dividing by small
constants especially if you knew your dividend was of less than the
full range of the possible register values.