Subject: Re: div by 10 on Z80

[Jim Battle]

dwight elvey wrote:
>> From: korpela at ssl.berkeley.edu
>>
>>> 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.
>>
> 
> 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.

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