How to reverse bits...

[Jon Brinkmann]

In article <9008241924.AA00274 at bisco.kodak.COM> nobody at Kodak.COM writes:
#> >This might be trivial, but here goes...
#> >What's the most optimal way to reverse the bits in an unsigned char,
#> >i.e. change from MSB to LSB ordering ?
#
#  I am looking at doing just this!  We are working on Sun 3/80 systems and
#porting to a Sparc station.  In the process, we are thinking of what might
#happen if we wanted to go to some other system (read that, "IBM") which uses
#the "wrong" byte order.

The sender isn't talking about BYTE order, he's talking about BIT order.
They are obviously NOT the same!  Byte order is trivially handled on most
machines.  There was an extensive discussion in this newsgroup about a year
ago.  Check the archives.

Now about bit order.  Most processors don't handle bits individually.  You'll
have to check each bit in the source and set the appropriate bit in the
destination.  Something like

	for (i = dest = 0, j = 1; i < 8; i++, j << 1)
		if (source & j) dest |= 1 << (7 - i);

Jon
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Jon Brinkmann					Astronomy Department
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