Hi Ash,

Thanks for replying, but I hope you can help me further. Actually all i
wanted to do is to write a module for CRC-16, if I cannot move on from that
section of Ross Williams notes, I will just find some prewritten modules and
transfered it to the current assembly I am using. (Mot) But I am trying hard
to understand how the algorithm works.

The earlier portion of the notes details all the arithmetic needed for CRC
calculation. I understand how the checksum ( crc ) is calculated, what is a
poly and the message that is being divided. When it comes to table driven
implementation, it explains how one byte can be XOR with a precalculated
byte instead of doing it bit by bit.

The C codes example is actually for a CRC 32 bit implementation. The ' r '
is a 4 byte register, the table[x] also points to a 4 byte contents. The
size of the table is [ 0 - 255 ].
The algorithm is as below -

1. Shift the register left by one byte, reading in a new message byte.
2. Use the top byte just rotated out of the register to index the table of
256 32 bit values.
3. XOR the table value into the register.
4. Goto 1 if more augmented message bytes.
End.

I could not see the flow of the calculation,
- why is the shifted byte use as an off set to the table for XOR purpose?
- after XORing, the register is suppose to contain the remainder, but why is
only a single byte shifted in/out to the register?

Forgive me for my messy questions, I just can't see the flow ( the way
normal CRC division being done ). I will appreciate if you can point me to
any manual workings of such an implementation.

Thanks and have a nice day ahead.

Rgds,

pang

----- Original Message -----
From: "Ashley Roll" <ash@DIGITALNEMESIS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, May 06, 2002 4:12 PM
Subject: Re: [OT]:Question on CRC ( based on notes by Ross Williams )


> Hi Pang,
>
> I've not looked at these notes, but I can help with the C..
>
> I'm assuming that "p" is pointing to the message being CRCed..
>
> > r = 0;
> > while ( len-- )
> > {
> >     byte t = ( r >> 24 ) & 0xFF;
>
> >     r = ( r << 8 )  | *p++;             // why OR with the
> > contents of the
> > address pointer?
>
> This is rotating the CRC value currently stored in "r" by 8 bits and then
it
> assigns the current message byte into the lower 8 bits of "r" and then
moves
> the pointer to point to the next element in the message.
>
> This is just a shorthand way of writing:
>
>         r = r << 8;             // rotate the CRC one byte left
>         r = r | (*p);   // set the lower byte to the current message value
>                                 // without changing the upper bytes of "r"
>         p++;                    // increment the pointer to the next byte
in the message
>
> >
> >     r^ =table[t];
> >
>
> This is XORing the value in "r" with the result of the table lookup.
>
> In "c", you can use the shorthand of X [operator]= value; to mean X = X
> [operator] value;
>
> The "^" means XOR in C.
>
> Does that help? Can you see now where "r" is being updated?
>
> If your going to port this to a PIC, remember - you will have different
size
> integers on the PIC. This looks like it is working with 32 bit ints.
>
> I've implemented a CRC16 in a PIC with table lookups before. You need two
> tables because it can't implement a table with a 16 bit value, so you need
a
> table for the "high" and "low" bytes.
>
> Cheers,
> Ash.
>
> ---
> Ashley Roll
> Digital Nemesis Pty Ltd
> www.digitalnemesis.com
> Mobile: +61 (0)417 705 718
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
>



> > -----Original Message-----
> > From: pic microcontroller discussion list
> > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Pang
> > Sent: Sunday, May 05, 2002 7:40 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: [OT]:Question on CRC ( based on notes by Ross Williams )
> >
> >
> > Hi all,
> >
> > I am currently reading some notes with the title 'CRC primer by Ross
> > Williams'. I am stuck at section 9-A table driven implementation.
> > Wondering
> > if anyone has gone through the same notes and hopefully provide me some
> > guidance.
> > The link for the notes are as below - www.riccibitti.com/crcguide.htm
> >
> > I do not understand what the C codes trying to do -
> >
> > r = 0;
> > while ( len-- )
> > {
> >     byte t = ( r >> 24 ) & 0xFF;
> >     r = ( r << 8 )  | *p++;             // why OR with the contents of
the
> > address pointer?
> >     r^ =table[t];
> > }
> >
> > where r = register, len = length of the message, t = temporary?,
> > p = points
> > to the augmented message
> >
> > I understand roughly the table way of implementation, but in the
> > code above,
> > r was not loaded with a new value, so how does the bytes move on?
> >
> > I will be gratefull for any guidance given.
> >
> > Rgds,
> >
> > Pang
> >
> > --

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