Path: utzoo!attcan!uunet!husc6!rutgers!deimos!uxc!uxc.cso.uiuc.edu!m.cs.uiuc.edu!wsmith
From: wsmith@m.cs.uiuc.edu
Newsgroups: comp.arch
Subject: Re: Leading zero and pop count
Message-ID: <3300039@m.cs.uiuc.edu>
Date: 19 Nov 88 18:11:00 GMT
References: <10882@hall.cray.com>
Lines: 23
Nf-ID: #R:hall.cray.com:10882:m.cs.uiuc.edu:3300039:000:646
Nf-From: m.cs.uiuc.edu!wsmith    Nov 19 12:11:00 1988


>One thing that is difficult to do in software (counter examples are welcomed
>please!) and easy to do in hardware is to reverse a bit stream.

What exactly to you mean by bit stream?   If you mean a single word, it is
very easy.  If it is more than a word the same trick may be used on a larger
scale:  (This is actually Pete Madany's code on loan.)

union WC {
	unsigned int	w;
	unsigned char	c[4];
} in, out;

out.c[3] = table[in.c[0]];
out.c[2] = table[in.c[1]];
out.c[1] = table[in.c[2]];
out.c[0] = table[in.c[3]];

The 256 byte array, table,  begins { 0x00, 0x80, 0x40, 0xc0, 0x20, 0xa0 ... }

Bill Smith
wsmith@cs.uiuc.edu
uiucdcs!wsmith