# Скачать CRC-32 Calculating Unit for Turbo Pascal

17.04.1988
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```Converted to Turbo Pascal (tm) V4.0 March, 1988 by J.R.Louvau
Copyright (C) 1986 Gary S. Brown.  You may use this program, or
code or tables extracted from it, as desired without restriction.

First, the polynomial itself and its table of feedback terms.  The
polynomial is
X^32+X^26+X^23+X^22+X^16+X^12+X^11+X^10+X^8+X^7+X^5+X^4+X^2+X^1+X^0
Note that we take it "backwards" and put the highest-order term in
the lowest-order bit.  The X^32 term is "implied"; the LSB is the
X^31 term, etc.  The X^0 term (usually shown as "+1") results in
the MSB being 1.

Note that the usual hardware shift register implementation, which
is what we're using (we're merely optimizing it by doing eight-bit
chunks at a time) shifts bits into the lowest-order term.  In our
implementation, that means shifting towards the right.  Why do we
do it this way?  Because the calculated CRC must be transmitted in
order from highest-order term to lowest-order term.  UARTs transmit
characters in order from LSB to MSB.  By storing the CRC this way,
we hand it to the UART in the order low-byte to high-byte; the UART
sends each low-bit to hight-bit; and the result is transmission bit
by bit from highest- to lowest-order term without requiring any bit
shuffling on our part.  Reception works similarly.

The feedback terms table consists of 256, 32-bit entries.  Notes:

The table can be generated at runtime if desired; code to do so
is shown later.  It might not be obvious, but the feedback
terms simply represent the results of eight shift/xor opera-
tions for all combinations of data and CRC register values.

The values must be right-shifted by eight bits by the "updcrc"
logic; the shift must be unsigned (bring in zeroes).  On some
hardware you could probably optimize the shift in assembler by
using byte-swap instructions.
polynomial \$edb88320

Use a type LONGINT variable to store the crc value.
Initialise the variable to \$FFFFFFFF before running the crc routine.
VERY IMPORTANT!!!! -> This routine was developed for data communications
and returns the crc bytes in LOW to HIGH order, NOT byte reversed!
To turn the valu into a 'normal' LONGINT, you must reverse the bytes!
e.g.

VAR
l, crc: LONGINT;
list: ARRAY[0..1023] OF BYTE;
counter: INTEGER;

BEGIN
crc := \$FFFFFFFF;                           (* initialise  *)
FillChar(list,SizeOf(list),1);              (* dummy array *)
FOR counter := 0 TO (Pred(SizeOf(list))) DO (* run thru    *)
crc := UpdC32(buf[counter],crc);         (* finding crc *)
FOR counter := 1 TO 4 DO                    (* reverse     *)
l := (l SHL 8) OR BYTE(crc);             (* the bytes   *)
(* l now contains the 'normalized' crc *)

FUNCTION UpdC32(octet: BYTE; crc: LONGINT) : LONGINT;
```