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1 /******************************************************************************/
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2 /* Start of crcmodel.c */
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3 /******************************************************************************/
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4 /* */
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5 /* Author : Ross Williams (ross@guest.adelaide.edu.au.). */
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6 /* Date : 3 June 1993. */
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7 /* Status : Public domain. */
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8 /* */
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9 /* Description : This is the implementation (.c) file for the reference */
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10 /* implementation of the Rocksoft^tm Model CRC Algorithm. For more */
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11 /* information on the Rocksoft^tm Model CRC Algorithm, see the document */
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12 /* titled "A Painless Guide to CRC Error Detection Algorithms" by Ross */
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13 /* Williams (ross@guest.adelaide.edu.au.). This document is likely to be in */
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14 /* "ftp.adelaide.edu.au/pub/rocksoft". */
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15 /* */
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16 /* Note: Rocksoft is a trademark of Rocksoft Pty Ltd, Adelaide, Australia. */
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17 /* */
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18 /******************************************************************************/
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19 /* */
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20 /* Implementation Notes */
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21 /* -------------------- */
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22 /* To avoid inconsistencies, the specification of each function is not echoed */
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23 /* here. See the header file for a description of these functions. */
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24 /* This package is light on checking because I want to keep it short and */
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25 /* simple and portable (i.e. it would be too messy to distribute my entire */
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26 /* C culture (e.g. assertions package) with this package. */
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27 /* */
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28 /******************************************************************************/
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29
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30 #include "crcmodel.h"
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31
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32 /******************************************************************************/
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33
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34 /* The following definitions make the code more readable. */
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35
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36 #define BITMASK(X) (1L << (X))
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37 #define MASK32 0xFFFFFFFFL
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38 #define LOCAL static
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39
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40 /******************************************************************************/
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41
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42 LOCAL ulong reflect P_((ulong v,int b));
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43 LOCAL ulong reflect (ulong v,int b)
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44 /* Returns the value v with the bottom b [0,32] bits reflected. */
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45 /* Example: reflect(0x3e23L,3) == 0x3e26 */
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46 {
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47 int i;
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48 ulong t = v;
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49 for (i=0; i<b; i++)
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50 {
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51 if (t & 1L)
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52 v|= BITMASK((b-1)-i);
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53 else
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54 v&= ~BITMASK((b-1)-i);
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55 t>>=1;
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56 }
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57 return v;
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58 }
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59
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60 /******************************************************************************/
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61
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62 LOCAL ulong widmask P_((p_cm_t));
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63 LOCAL ulong widmask (p_cm_t p_cm)
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64 /* Returns a longword whose value is (2^p_cm->cm_width)-1. */
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65 /* The trick is to do this portably (e.g. without doing <<32). */
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66 {
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67 return (((1L<<(p_cm->cm_width-1))-1L)<<1)|1L;
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68 }
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69
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70 /******************************************************************************/
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71
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72 void cm_ini (p_cm_t p_cm)
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73 {
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74 p_cm->cm_reg = p_cm->cm_init;
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75 }
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76
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77 /******************************************************************************/
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78
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79 void cm_nxt (p_cm_t p_cm, int ch)
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80 {
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81 int i;
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82 ulong uch = (ulong) ch;
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83 ulong topbit = BITMASK(p_cm->cm_width-1);
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84
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85 if (p_cm->cm_refin) uch = reflect(uch,8);
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86 p_cm->cm_reg ^= (uch << (p_cm->cm_width-8));
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87 for (i=0; i<8; i++)
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88 {
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89 if (p_cm->cm_reg & topbit)
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90 p_cm->cm_reg = (p_cm->cm_reg << 1) ^ p_cm->cm_poly;
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91 else
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92 p_cm->cm_reg <<= 1;
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93 p_cm->cm_reg &= widmask(p_cm);
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94 }
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95 }
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96
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97 /******************************************************************************/
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98
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99 void cm_blk (p_cm_t p_cm,p_ubyte_ blk_adr,ulong blk_len)
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100 {
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101 while (blk_len--) cm_nxt(p_cm,*blk_adr++);
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102 }
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103
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104 /******************************************************************************/
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105
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106 ulong cm_crc (p_cm_t p_cm)
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107 {
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108 if (p_cm->cm_refot)
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109 return p_cm->cm_xorot ^ reflect(p_cm->cm_reg,p_cm->cm_width);
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110 else
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111 return p_cm->cm_xorot ^ p_cm->cm_reg;
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112 }
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113
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114 /******************************************************************************/
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115
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116 ulong cm_tab (p_cm_t p_cm, int index)
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117 {
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118 int i;
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119 ulong r;
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120 ulong topbit = BITMASK(p_cm->cm_width-1);
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121 ulong inbyte = (ulong) index;
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122
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123 if (p_cm->cm_refin) inbyte = reflect(inbyte,8);
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124 r = inbyte << (p_cm->cm_width-8);
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125 for (i=0; i<8; i++)
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126 if (r & topbit)
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127 r = (r << 1) ^ p_cm->cm_poly;
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128 else
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129 r<<=1;
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130 if (p_cm->cm_refin) r = reflect(r,p_cm->cm_width);
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131 return r & widmask(p_cm);
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132 }
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133
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134 /******************************************************************************/
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135 /* End of crcmodel.c */
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136 /******************************************************************************/
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137
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