crc4.c

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/*
 * crc4.c 
 * 
 * functions for 4-bit CRC calculations
 * assumes no more than 12 info. bits
 * Max Luttrell 8/19/96
 */

#include "standard.h"
#include "bits.h"

#define GENPOLY 0x0017 /* x^4 + x^2 + x + 1 */

unsigned int makecrc4(unsigned int b)
/*
 * Takes b as input, which should be the information vector
 * already multiplied by x^4 (ie. shifted over 4 bits), and
 * returns the crc for this input based on the defined generator
 * polynomial GENPOLY
 */
{
  int i;
        
  i=1;
  while (b>=16) { /* >= 2^4, so degree(b) >= degree(genpoly) */
    if (getbitint(b,i) == 1)
      b ^= GENPOLY << (12-i); /* reduce with GENPOLY */
    i++;
  }
  return b;
}

int checkcrc4(unsigned int b)
/*
 * Takes b as input, and determines if it passes CRC check.
 * It does this by finding out if it is a multiple of GENPOLY.
 * Returns 1 if passes, 0 if fails.
 */
{
  int i;
        
  i=1;
  while (b>=16) { /* >= than 2^4, so degree(b) >= degree(genpoly) */
  if (getbitint(b,i) == 1)
    b ^= GENPOLY << (12-i); /* reduce with GENPOLY */
  i++;
  }
/*  printf("b: %d\n",b); */
  return (b==0);
}

#if(0)
main()
{
        unsigned int data;
        byte crc;

        data = 0x06c0;
        crc = makecrc4(data);
        printf("crc : %x, data : %x\n",crc,data);
        data=data | crc;
        printf("data after OR : %x\n",data);
        /* data = data | 0x0080; */
        data = 0x06c7;
        printf("data after corruption : %x\n",data);

        printf("the crc check...%d\n",checkcrc4(data));
}
#endif

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