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Diffstat (limited to 'ax25ipd/crc.c')
| -rw-r--r-- | ax25ipd/crc.c | 171 |
1 files changed, 171 insertions, 0 deletions
diff --git a/ax25ipd/crc.c b/ax25ipd/crc.c new file mode 100644 index 0000000..ff07195 --- /dev/null +++ b/ax25ipd/crc.c @@ -0,0 +1,171 @@ +/* crc.c Computations involving CRCs */ + +#include "ax25ipd.h" +/* + ********************************************************************** + * The following code was taken from Appendix B of RFC 1171 + * (Point-to-Point Protocol) + * + * The RFC credits the following sources for this implementation: + * + * Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983. + * + * Morse, G., "Calculating CRC's by Bits and Bytes", Byte, + * September 1986. + * + * LeVan, J., "A Fast CRC", Byte, November 1987. + * + * + * The HDLC polynomial: x**0 + x**5 + x**12 + x**16 + */ + +/* + * u16 represents an unsigned 16-bit number. Adjust the typedef for + * your hardware. + */ +typedef unsigned short u16; + + +/* + * FCS lookup table as calculated by the table generator in section 2. + */ +static u16 fcstab[256] = { + 0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf, + 0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7, + 0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e, + 0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876, + 0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd, + 0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5, + 0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c, + 0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974, + 0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb, + 0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3, + 0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a, + 0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72, + 0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9, + 0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1, + 0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738, + 0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70, + 0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7, + 0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff, + 0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036, + 0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e, + 0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5, + 0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd, + 0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134, + 0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c, + 0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3, + 0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb, + 0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232, + 0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a, + 0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1, + 0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9, + 0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330, + 0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78 +}; + +#define PPPINITFCS 0xffff /* Initial FCS value */ +#define PPPGOODFCS 0xf0b8 /* Good final FCS value */ + +/* + * Calculate a new fcs given the current fcs and the new data. + */ +u16 pppfcs(fcs, cp, len) + register u16 fcs; + register unsigned char *cp; + register int len; +{ +/* ASSERT(sizeof (u16) == 2); */ +/* ASSERT(((u16) -1) > 0); */ + while (len--) + fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff]; + + return (fcs); +} + +/* + * End code from Appendix B of RFC 1171 + ********************************************************************** + */ + +/* + * The following routines are simply convenience routines... + * I'll merge them into the mainline code when suitably debugged + */ + +/* Return the computed CRC */ +unsigned short int +compute_crc(buf, l) +unsigned char *buf; +int l; +{ + int fcs; + + fcs = PPPINITFCS; + fcs = pppfcs(fcs, buf, l); + fcs ^= 0xffff; + return fcs; +} + +/* Return true if the CRC is correct */ +int +ok_crc(buf, l) +unsigned char *buf; +int l; +{ + int fcs; + + fcs = PPPINITFCS; + fcs = pppfcs(fcs, buf, l); + return (fcs == PPPGOODFCS); +} + +/* + * A test routine to make sure the CRC is working right on your hardware. + * cc -DTEST crc.c + * + */ + +#ifdef TEST +void +main() +{ + unsigned char buf[258]; + int l, i; + unsigned short int f; + + l = 256; + for(i=0;i<l;i++){ + buf[i]=i; + } + + f = compute_crc(buf,l); + printf("computed crc=0x%04x\n",f); + + buf[l]=(f&0xff); + buf[l+1]=(f>>8); + printf("crc should be good... "); + i = ok_crc(buf, l+2); + if(i)printf("CRC declared OK\n"); + else printf("CRC declared bad\n"); + + buf[l+1]=(f&0xff); + buf[l]=(f>>8); + printf("reversed the CRC byte order... CRC should be bad..."); + i = ok_crc(buf, l+2); + if(i)printf("CRC declared OK\n"); + else printf("CRC declared bad\n"); + + printf("changed the frame length... CRC should be bad..."); + i = ok_crc(buf, l+1); + if(i)printf("CRC declared OK\n"); + else printf("CRC declared bad\n"); + + buf[0]-=1; + printf("corrupted the data... CRC should be bad..."); + i = ok_crc(buf, l+2); + if(i)printf("CRC declared OK\n"); + else printf("CRC declared bad\n"); + +} +#endif |