J.C. Wren wrote:
I first saw this on Steve's Cute Code Collection (<
http://www.sjbaker.org/steve/software/cute_code.html >). He says he
found the 32 bit version in the *nix fortune cookie program. This
method is pretty effective for 32 bit numbers, since the look up table
version can get a little large :)
For 8 bit numbers, using Steve's shift left/shift right method, I
don't think I'd bother with a looping structure. Better just to inline
the 16 instructions. The difference in code savings will be minimal and
the execution speed won't change much, but it will eliminate managing an
extra register for the loop.
gcc decided to unroll the loop into the 16 shifts anyway. C code with loop is
nicer to read - i think.
Georg
There's a few other tricks on the page that can be handy, also.
--jc
Steve Underwood wrote:
Hi,
Gosh, I haven't seen that one for years. I think it needs a barrel
shifter to work well, though. I'm pretty sure direct shift out and
shift in will do better on the MSP430.
Regards,
Steve
Regards,
Steve
Leon Heller wrote:
Here's a clever way to do it that I found via Google:
//
// Reverse the order of bits within a byte.
// Returns: The reversed byte value.
//
BYTE ReverseBits(BYTE b)
{
BYTE c;
c = ((b >> 1) & 0x55) | ((b << 1) & 0xaa);
c |= ((b >> 2) & 0x33) | ((b << 2) & 0xcc);
c |= ((b >> 4) & 0x0f) | ((b << 4) & 0xf0);
return(c);
}
I haven't checked it, though.
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