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Date : Tue Jun 16, 13:58                                                       
From : Andrey Pashchenko                                       2:461/121
To   : All                                 
Subj : CRC, ¢ ⮬ ç¨á«¥ ¤«ï ®¤­®ªà¨áâ «®ª.                                   
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From: "Andrey Pashchenko" <aapash@geocities.com>


The Cyclical Redundancy Check (CRC) field is two bytes, containing
a 16-bit binary value.

   ...

The CRC is started by first preloading a 16-bit register to all 1's.
Then a process begins of applying successive 8-bit bytes of the
message to the current contents of the register. Only the eight bits
of data in each character are used for generating the CRC. Start and
stop bits, and the parity bit if one is used, do not apply to the CRC.

During generation of the CRC, each 8-bit character is exclusive
ORed with the register contents. Then the result is shifted in the
direction of the least significant bit (LSB), with a zero filled into
the most significant bit (MSB) position. The LSB is extracted and
examined. If the LSB was a 1, the register is then exclusive ORed
with a preset, fixed value. If the LSB was a 0, no exclusive OR
takes place.

This process is repeated until eight shifts have been performed.
After the last (eighth) shift, the next 8-bit character is
exclusive ORed with the register's current value, and the process
repeats for eight more shifts as described above. The final
contents of the register, after all the characters of the message
have been applied, is the CRC value.

A procedure for generating a CRC is:

1. Load a 16-bit register with FFFF hex (all 1's). Call this
the CRC register.

2. Exclusive OR the first 8-bit byte of the message with the
low-order byte of the 16-bit CRC register, putting the result in
the CRC register.

3. Shift the CRC register one bit to the right (toward the LSB),
zero-filling the MSB. Extract and examine the LSB.

4. (If the LSB was 0):   Repeat Step 3 (another shift).
   (If the LSB was 1):   Exclusive OR the CRC register with the
         polynomial value A001 hex (1010 0000 0000 0001).

5. Repeat Steps 3 and 4 until 8 shifts have been performed.
   When this is done, a complete 8-bit byte will have been processed.

6. Repeat Steps 2 through 5 for the next 8-bit byte of the message.
   Continue doing this until all bytes have been processed.

7. The final contents of the CRC register is the CRC value.

   ...

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