> From:    Scott Dattalo <sdattalo@UNIX.SRI.COM>
> Subject: Shift right (or left) 4
>
> How fast does your favorite shifting program shift a 32-bit unsigned integer
> four bit positions? My first attempt yielded a 22 instruction slug, while my
> second attempt is a 19 instruction screamer. Anybody have any idea on how to
> make it scream louder?

I don't know if you are trying to optimise size or speed.  Luckily
my simple solution improves on both.  I haven't tested this!

        rrf hh, F       ; move the whole chain down one bit,
        rrf mh, F       ;   / using the carry as temporary storage
        rrf ml, F       ;  /
        rrf ll, F       ; /

        (repeat the above code three more times, and then)

        ; now, the top byte has four bits of rubbish at the
        ; top, so we have to clear them:

        movlw 0xF       ; bit mask in W
        andwf hh, F     ; set top four bits to 0

        Total 18 words.

If you are going for size optimisation, put the repeated section in
a loop!

Andrew
Sharp Labs Europe Ltd, Oxford Science Park, Oxford, UK, OX4 4GA
Andrew.Kay@sharp.co.uk  Tel:+44 1865 747711 FAX:+44 1865 747717