**Claus-Justus Heine** (*claus@momo.math.rwth-aachen.de*)

*18 Jun 1998 15:07:25 +0200*

**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]**Next message:**Marcus Berglund: "[PARPORT] Massive slowdown of computer during transfer"**Previous message:**Claus-Justus Heine: "[PARPORT] Re: Iomega Ditto 3200"**In reply to:**Claus-Justus Heine: "[PARPORT] Re: Iomega Ditto 3200"

Claus-Justus Heine <claus@momo.math.rwth-aachen.de> writes:

*> Claus-Justus Heine <claus@momo.math.rwth-aachen.de> writes:
*

*>
*

*> > Mmmh. Well. Actually: it should work to use gzip's way of computing
*

*> > things, and then multiply the resulting CRC by x^15 and take the
*

*> > remainder mod x^16.
*

*>
*

*> The three lines above are complete nonsense. I'm sorry.
*

Ok, next attempt. Of course, reverting the order of powers in a

polynomial ring IS an isomorphism. And the connection between the two

point of views (i.e. LSB is the highest power of x <-> MSB is the

highest power of x) w.r.t. to computing a CRC sum really is only a

matter of reverting the order of bits. I.e. a generating polynomial

0x1021 (that is x^0 + x^5 + x^12 + x^16) where the LSB corresponds to

the coefficient of x^0, generates the same CRC values as the bit

reverted polynomial 0x8408 (which also is x^0 + x^5 + x^12 + x^16),

only that the order of bits in the resulting CRC sums is reverted

(w.r.t. to the other point of view).

The included sample program below computes both case for some 32 byte

sample input streams.

Compile with gcc -o crc crc.c

and run with

crc 0x8408

or

crc reverse 0x1021

to get the result the bpck tape drive uses.

Cheers

Claus

Claus-Justus Heine

claus@momo.math.rwth-aachen.de

http://www-math.math.rwth-aachen.de/~LBFM/claus

PGP Public Key:

http://www-math.math.rwth-aachen.de/~LBFM/claus/claus-public-key.asc

Ftape - the Linux Floppy Tape Project

WWW : http://www-math.math.rwth-aachen.de/~LBFM/claus/ftape

Mailing-list: linux-tape@vger.rutgers.edu

- application/octet-stream attachment: crc.c

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