🚀 go-pugleaf

In article <1992Aug28.083405.164422@dstos3.dsto.gov.au> egg@dstos3.dsto.gov.au writes:
>Does anyone know how to quickly and efficiently mirror a byte.
>ie
>    11001010 to 01010011
>
>the only way I know is this
>
>   mov  al,byte_to_shift
>   mov  cx,8
>ShiftLoop:
>   rcr  al
>   rcl  ah
>   loop ShiftLoop
>   mov  shifted_byte,ah
>
>There must be an easier (and smarter) way to speed this up using logical
>op's perhaps ?
>Any Help appreciated.
>
>Themie Gouthas
>egg@dstos3.dsto.gov.au


The fastest way of doing this is to use look up tables.
You could make a look up table for the nibbles. 0000 to 1111. That's
just 16 bytes. You could squeeze it into 8 bytes, but the code
would be longer and it would take more time. Then to mirror a byte,
you just look up the two nibbles, and swap them. This procedure can
be used for words etc. of course.

I don't think this algorithm including the table would take much
more space than yours.

If time is more important than size, use a look up table for the
bytes.



-- 
Stig (venaas@siri.unit.no)