Hi!

El Wed, Mar 10, 2004 at 09:40:28PM +0100, Wouter van Ooijen escribio:
> Someone asked me for a fast yet not extremely large code snipped to
> divide a one-byte value in memory by 7 (on a 14-bit core). I think I
> have a straight-line (no loop) code that uses 33 instructions (2 less if
> the source can be destroyed). Looping version would be 12 instructions,
> but of course somewhat longer execution. Can anyone do it faster,
> preferrably with fewer instructions? I know a table lookup will be
> faster, but I think the 256+ size would be classified as 'extremely
> large'.

You could try coding:

  div = (x*8 + x + x/4 - x/16 - x/64)/64

It gives correct results up to x=285. But it needs an 11-bit
acumulator.

I think that it's faster coding using 12bit acumulator:

  div = (x*16 + x*2 + x/2 - x/8 )/128    (acurate only up to 201)

or
  div = (x*16 + x*2 + x/2 - x/8 - x/16) / 128  (acurate up to 663)

The later has x*16 and x/16, that are easily obtained by swap's.

Other formulas can be derived easily...

    Daniel.

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