Use the following algorithm:
low_nibble = input_number & 0x0F;
low_nibble_rev = (bit_reverse_lookup[low_nibble]) <<
4;
hi_nibble = input_number >>4;
hi_nibble_rev = bit_reverse_lookup[hi_nibble];
answer = hi_nibble_rev | low_nibble_rev;
Note that bit_reverse_lookup array is 16 bytes in
size.
I am not sure if this version is any more efficient
than doing the bit reverse bit by bit. But does
require smaller lookup table.
If you can spare 16 more bytes than you can make the
above algorithm slightly more efficient:
low_nibble = input_number & 0x0F;
low_nibble_rev = bit_reverse_lookup_lo[low_nibble];
hi_nibble = input_number >>4;
hi_nibble_rev = bit_reverse_lookup_hi[hi_nibble];
answer = hi_nibble_rev | low_nibble_rev;
-Sumukh
--- David Brown <da...@westcontrol.com> wrote:
>
>
> > Hi,
> >
> > thanks for all the input!
> > Leon, this really is a cool function. It will go
> to my collection ;-).
> >
> > Though I used your version, Steve. Worked out of
> the box!
> > Why does it say "+r" ?
>
> I think the "+r" means that it is both an input and
> an output, allowing it
> to be initialised at the C level ("unsigned char tmp
> = 0"). Of course, this
> initialisation is not actually needed, since all 8
> bits of the original tmp
> get shifted out, so an instruction could be saved by
> using plain "unsigned
> char tmp;", and changing to "=r".
>
> David
>
>
> > And, perhaps you could add the "gcc manual hint"
> to the 1.ยง in chapter 6
> (didn't
> > even had the idea to look there, too).
> >
> > Greetz and thanks,
> >
> > Georg
> >
> >
> > Steve Underwood wrote:
> > > Hi Georg,
> > >
> > > I haven't tested this, but to embed the assembly
> language you need
> > > something like the following. If you look in the
> mspgcc documentation
> > > you will find some information about this. If
> you look in the GNU GCC
> > > documentation you will find some more. However,
> this area is not as well
> > > documented as it might be, with clear examples,
> etc. Figuring out
> > > exactly how to use those dependency fields is
> not easy.
> > >
> > > Regards,
> > > Steve
> > >
> > > unsigned char reverse(unsigned char b)
> > > {
> > > unsigned char tmp = 0;
> > >
> > > for( i=0; i<8; i++ )
> > > {
> > > __asm__ (
> > > " rrc.b %[b] \n"
> > > " rrl.b %[tmp] \n"
> > > : [tmp] "+r"(tmp)
> > > : [b] "r"(b));
> > > }
> > > return tmp;
> > > }
> > >
> > > Georg Ritter wrote:
> > >
> > >> Good morning!
> > >>
> > >> I'm looking for a proper implementation of a
> bitwisereverse function
> > >> of a byte (1010 0001 becomes: 1000 0101). I
> want to replace an ugly
> > >> and clumpsy function I used in testing.
> > >>
> > >> I would like to write smth like:
> > >>
> > >> unsigned char reverse(unsigned char b)
> > >> {
> > >> unsigned char tmp=0;
> > >> for( i=0; i<8; i++ )
> > >> {
> > >> // RRC.B of b
> > >> // RRL.B into tmp
> > >> }
> > >> return tmp;
> > >> }
> > >>
> > >> There's no real "C" way of doing it, isn't it?
> So what could the
> > >> inline asmbly line for that look like. I never
> used it before, and
> > >> it's tricky and documents didn't enlighten my
> too much. So my versions
> > >> either didn't compile or didn't work.
> > >>
> > >> Or is there a smaller (in terms of code size)
> way of doing the
> > >> reversion (hacker contest ;-) )?
> > >..
> >
> >
> >
> >
> >
>
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> >
> >
> >
>
>
>
>
>
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