Скачать TPZcrc - CRC16 and CRC32 routines for Turbo Pascal

20.05.1988
Скачать файл (12,38 Кб)

* UPDCRC derived from article Copyright (C) 1986 Stephen Satchell.
 * crctab calculated by Mark G. Mendel, Network Systems Corporation
 *
 *  NOTE: First argument must be in range 0 to 255.
 *        Second argument is referenced twice.
 *
 * Programmers may incorporate any or all code into their programs,
 * giving proper credit within the source. Publication of the
 * source routines is permitted so long as proper credit is given
 * to Stephen Satchell, Satchell Evaluations and Chuck Forsberg,
 * Omen Technology.
 *


 * UpdCRC32
 * 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