CRC

[Kamen Penev]

Carlos,

Depending on the generative polynomial you can get different formulas 
for computing a CRC. I am attaching a program that generates several 
popular varieties of CRC routines. I've used a couple of them 
successfully. 

I don't know who the author is. I got this off the Net.

But before you go any further, see if the debug monitor (if you have 
one) on your board already provides a CRC routine. Many do.

Kamen

----------------------
/* This program will write six C routines for the calculation of
 * the following CRC's. */

/* The CRC polynomial.
 * These 4 values define the crc-polynomial.
 * If you change them, you must change crctab[]'s initial value to what is
 * printed by initcrctab() [see 'compile with -DMAKETAB' above].
 */

/* This tables assumes CCITT is MSB first.  Swapped means LSB first.   
In that
 * case the polynomial is also swapped
 */

/* 16 bit crc's */
/* Value used by:            CCITT    KERMIT    ARC    BINHEX    */
/* the poly:                0x1021    0x8408    0xA001    0x1021    */
/* original:                0x1021    0x1021    0x8005    0x1021    */
/* init value:                -1    0    0    0    */
/* swapped:                no    yes    yes    no    */
/* bits in CRC:                16    16    16    16    */
/* ARC used by LHARC, ZOO, STUFFIT                    */
/* BINHEX used by XMODEM, PACKIT                    */

/* 32 bit crc's */
/* Value used by:            CCITT32        ZIP        */
/* the poly:                0x04c11db7    0xedb88320    */
/* original:                0x04c11db7    0x04c11db7    */
/* init value:                -1        -1        */
/* swapped                no        yes        */
/* bits in CRC:                32        32        */
/* ZIP used by COMPACTOR                        */

#include <stdio.h>

extern void exit();
extern char *strcat();

static void initcrctab();

main()
{
    initcrctab("ccitt", 0x1021, 0xffff, 0, 16);
    initcrctab("kermit", 0x8408, 0, 1, 16);
    initcrctab("arc", 0xa001, 0, 1, 16);
    initcrctab("binhex", 0x1021, 0, 0, 16);
    initcrctab("ccitt32",0x04c11db7,0xffffffff,0,32);
    initcrctab("zip",0xedb88320,0xffffffff,1,32);
    exit(0);
    /*NOTREACHED*/
}

static void initcrctab(name, poly, init, swapped, bits)
char *name;
int poly, init, swapped, bits;
{
    register  int b, i;
    unsigned short v;
    unsigned long vv;
    FILE *fd;
    char buf[20];
   
    buf[0] = 0;
    (void)strcat(buf, name);
    (void)strcat(buf, ".c");
    if((fd = fopen(buf, "w")) == NULL) {
    (void)fprintf(stderr, "Cannot open %s for writing\n", buf);
    exit(1);
    }
    (void)fprintf(fd, "unsigned long %s_crcinit = %d;\n", name, init);
    (void)fprintf(fd, "\n");
    if(bits == 16) {
    (void)fprintf(fd, "static unsigned short crctab[256] = {\n");
    } else {
    (void)fprintf(fd, "static unsigned long crctab[256] = {\n");
    }
    (void)fprintf(fd, "    ");
    if(bits == 16) {
    for(b = 0; b < 256; ++b) {
        if(swapped) {
        for(v = b, i = 8; --i >= 0;)
            v = v & 1 ? (v>>1)^poly : v>>1;
        } else {
        for(v = b<<8, i = 8; --i >= 0;)
            v = v & 0x8000 ? (v<<1)^poly : v<<1;
        }
        (void)fprintf(fd, "0x%.4x,", v & 0xffff);
        if((b&7) == 7) {
        (void)fprintf(fd, "\n");
        if(b != 255) (void)fprintf(fd, "    ");
        } else {
        (void)fprintf(fd, " ");
        }
    }
    } else {
    for(b = 0; b < 256; ++b) {
        if(swapped) {
        for(vv = b, i = 8; --i >= 0;)
            vv = vv & 1 ? (vv>>1)^poly : vv>>1;
        } else {
        for(vv = b<<24, i = 8; --i >= 0;)
            vv = vv & 0x80000000 ? (vv<<1)^poly : vv<<1;
        }
        (void)fprintf(fd, "0x%.8x,", vv & 0xffffffff);
        if((b&3) == 3) {
        (void)fprintf(fd, "\n");
        if(b != 255) (void)fprintf(fd, "    ");
        } else {
        (void)fprintf(fd, " ");
        }
    }
    }
    (void)fprintf(fd, "};\n");
    (void)fprintf(fd, "\n");
    (void)fprintf(fd, "unsigned long %s_updcrc(icrc, icp, icnt)\n", name);
    (void)fprintf(fd, "    unsigned long icrc;\n");
    (void)fprintf(fd, "    unsigned char *icp;\n");
    (void)fprintf(fd, "    int icnt;\n");
    (void)fprintf(fd, "{\n");
    if(bits == 16) {
    (void)fprintf(fd, "#define M1 0xff\n");
    (void)fprintf(fd, "#define M2 0xff00\n");
    } else {
    (void)fprintf(fd, "#define M1 0xffffff\n");
    (void)fprintf(fd, "#define M2 0xffffff00\n");
    }
    (void)fprintf(fd, "    register unsigned long crc = icrc;\n");
    (void)fprintf(fd, "    register unsigned char *cp = icp;\n");
    (void)fprintf(fd, "    register int cnt = icnt;\n");
    (void)fprintf(fd, "\n");
    (void)fprintf(fd, "    while(cnt--) {\n");
    if(bits == 16) {
    if (swapped) {
        (void)fprintf(fd,
            "\tcrc=((crc>>8)&M1)^crctab[(crc&0xff)^*cp++];\n");
    } else {
        (void)fprintf(fd,
            "\tcrc=((crc<<8)&M2)^crctab[((crc>>8)&0xff)^*cp++];\n");
    }
    } else {
    if(swapped) {
        (void)fprintf(fd,
            "\tcrc=((crc>>8)&M1)^crctab[(crc&0xff)^*cp++];\n");
    } else {
        (void)fprintf(fd,
            "\tcrc=((crc<<8)&M2)^crctab[((crc>>24)&0xff)^*cp++];\n");
    }
    }
    (void)fprintf(fd, "    }\n");
    (void)fprintf(fd, "\n");
    (void)fprintf(fd, "    return(crc);\n");
    (void)fprintf(fd, "}\n");
    (void)fprintf(fd, "\n");
    (void)fclose(fd);
}