[Oberon] FPGA - Bit reversal

[Tomas Kral]

On Fri, 22 Sep 2017 12:22:06 +0300
Alexander Ilin <ajsoft at yandex.ru> wrote:

> PROCEDURE ReverseByte(wd: INTEGER): INTEGER;
>     VAR i: INTEGER; rbt, u: SET;
>   BEGIN
>     u := SYSTEM.VAL(SET, wd);
>     rbt := {}; FOR i := 0 TO 7 DO
>       IF i IN u THEN INCL(rbt, 7-i) END
>     END
>   RETURN SYSTEM.VAL(INTEGER, rbt) END ReverseByte;

Hi Alex (or prefer Sasha?),

This is good, it fully utilises Oberon SETs, I have not thought of
that :-)

How would you recommend to further optimise word bit reversal?

  PROCEDURE ReverseWord(wd: INTEGER): INTEGER;
    VAR i: INTEGER; rwd, u: SET;
  BEGIN
    u := SYSTEM.VAL(SET, wd);
    rwd := {}; FOR i := 0 TO 31  DO
      IF i IN u THEN INCL(rwd, 31-i) END
    END   
  RETURN SYSTEM.VAL(INTEGER, rwd) END ReverseWord;

I was also thinking to use WHILE loop and let i run from 0 to 15 and j
from 31 to 16. Not sure if faster with only 16 iterations.

`C' pseudo code gives this, if could be modified for array lookup, as
per Joerg's suggestion on BYTE type reversal.

unsigned int
reverse(register unsigned int x)
{
    x = (((x & 0xaaaaaaaa) >> 1) | ((x & 0x55555555) << 1));
    x = (((x & 0xcccccccc) >> 2) | ((x & 0x33333333) << 2));
    x = (((x & 0xf0f0f0f0) >> 4) | ((x & 0x0f0f0f0f) << 4));
    x = (((x & 0xff00ff00) >> 8) | ((x & 0x00ff00ff) << 8));
    return((x >> 16) | (x << 16));

}
     
Many thanks.

-- 
Tomas Kral <thomas.kral at email.cz>