Date   : Thu, 02 Jun 2005 13:50:41 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: Assembly Language Square Root

Michael Borcherds wrote:

> Peter Craven <cjl_craves@...> wrote:
>
> > I came across a Z80 Assembly Language program at the end of the
> > web page at: http://map.tni.nl/articles/mult_div_shifts.php
>
> > Sqr16_Loop:
> >  sbc hl,de
>
> The Z80 carry is used the other way round to the 6502 for the SBC
> instruction so you might need to think about swapping the carry
> before and after it.

Yes...

Other gotchas:

The LD DE,#0040 is a hex constant - the '#' denoting 'hex', not 'immediate
mode'.  (Z80 doesn't use # to indicate immediate - all constants are
immediate, and contents of memory locations are accessed by putting the
operand in brackets).  Hence your 6502 version should be doing
LDA#&40:STAd.

I'll have a go at 6502ing that code myself in a sec - I suspect that,
whereas it is tuned very nicely to a Z80, with its 16-bit add opcodes and
so on, it may be suboptimal on a 6502.  This is also true of Z80 multiply
routines which are able to use the obvious left-shift method to achieve
their goal, versus the 6502 versions which make very clever use of right
shifts instead.

Meanwhile, there's a square-root routine here (though I don't know how good
it is):

http://www.6502.org/source/integers/root.htm

Cheers,
Rich


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