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crc32.c
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crc32.c
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/*
* 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
*/
#include <common.h>
u32 crc32_table[256];
void make_crc_table()
{
unsigned long c;
int n, k;
unsigned long poly; /* polynomial exclusive-or pattern */
poly = 0xedb88320UL;
/* generate a crc for every 8-bit value */
for (n = 0; n < 256; n++) {
c = (unsigned long)n;
for (k = 0; k < 8; k++)
c = c & 1 ? poly ^ (c >> 1) : c >> 1;
crc32_table[n] = c;
}
}