5 #define EXTRA_BIT_POSITION (PARITY_BITS - 1)
6 #define CODE_BITS (DATA_BITS + PARITY_BITS)
7 #define NUM_DATA_WORDS (1 << DATA_BITS)
10 * Needed since we store all the parity at the end of the word, not at the expected
11 * power-of-two bit positions.
13 unsigned char permutation_table[CODE_BITS] = {
14 0, 5, 4, 31, 3, 30, 29, 28, 2, 27, 26, 25, 24, 23, 22, 21, 1, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6
17 unsigned find_parity_32(unsigned data)
19 data = (data >> 16) ^ data;
20 data = (data >> 8) ^ data;
21 data = (data >> 4) ^ data;
22 data = (data >> 2) ^ data;
23 data = (data >> 1) ^ data;
27 unsigned generate_parity(unsigned data)
29 unsigned parity1 = find_parity_32(data & 0x036ad555);
30 unsigned parity2 = find_parity_32(data & 0x02d9b333);
31 unsigned parity3 = find_parity_32(data & 0x01c78f0f);
32 unsigned parity4 = find_parity_32(data & 0x003f80ff);
33 unsigned parity5 = find_parity_32(data & 0x00007fff);
34 unsigned parity6 = find_parity_32(data & 0x03b4e996);
36 return parity6 | (parity5 << 1) | (parity4 << 2) | (parity3 << 3) | (parity2 << 4) | (parity1 << 5);
39 unsigned make_codeword(unsigned data)
41 return (data << PARITY_BITS) | generate_parity(data);
44 /* can detect all single or double bit errors */
45 int has_error(unsigned code)
47 unsigned data = code >> PARITY_BITS;
48 unsigned parity = code & ((1 << PARITY_BITS) - 1);
50 return (generate_parity(data) != parity);
53 int has_double_error(unsigned code)
56 unsigned data = code >> PARITY_BITS;
57 unsigned parity = code & ((1 << PARITY_BITS) - 1);
58 unsigned gen_parity = generate_parity(data);
60 unsigned hamming_parity = parity >> 1;
61 unsigned gen_hamming_parity = gen_parity >> 1;
62 unsigned extra_parity = find_parity_32(code);
64 /* no errors at all (user should have used has_error() first; boo, hiss) */
65 if (hamming_parity == gen_hamming_parity && extra_parity == 0)
68 /* both hamming and simple parity errors; this is a single-bit error */
69 if (hamming_parity != gen_hamming_parity && extra_parity == 1)
72 /* hamming says OK, but simple parity indicates an error => simple parity error is wrong */
73 if (hamming_parity == gen_hamming_parity && extra_parity == 1)
76 /* hamming says error, simple parity says OK => DOUBLE ERROR */
80 /* Correct any single-bit error -- assumes there are no double-bit errors */
81 unsigned correct_single_bit_error(unsigned code)
83 unsigned parity_diff = generate_parity(code >> PARITY_BITS) ^ code;
86 for (i = 0; i < PARITY_BITS - 1; ++i) {
87 if (parity_diff & (1 << (PARITY_BITS - 1 - i))) {
93 /* flip the wrong bit */
94 code ^= (1 << permutation_table[bp]);
97 /* recompute the lower parity */
98 return (code & ~1) | find_parity_32(code & ~1);
101 void check_zero_bit_detection()
104 printf("Checking zero bit detection.\n");
106 for (i = 0; i < NUM_DATA_WORDS; ++i) {
107 unsigned code = make_codeword(i);
108 if (has_error(code)) {
109 printf("ERROR: Failed zero-bit test 1 for %x\n", i);
111 if (has_double_error(code)) {
112 printf("ERROR: Failed zero-bit test 2 for %x\n", i);
117 void check_single_bit_detection()
120 printf("Checking single bit detection and correction.\n");
122 for (i = 0; i < NUM_DATA_WORDS; ++i) {
123 unsigned code = make_codeword(i);
124 for (j = 0; j < CODE_BITS; ++j) {
125 unsigned corrupted_code = code ^ (1 << j);
127 if (!has_error(corrupted_code)) {
128 printf("ERROR: Failed single-bit test 1 for %x with bit %u flipped\n", i, j);
130 if (has_double_error(corrupted_code)) {
131 printf("ERROR: Failed single-bit test 2 for %x with bit %u flipped\n", i, j);
133 if (correct_single_bit_error(corrupted_code) != code) {
134 printf("ERROR: Failed single-bit correction test for %x with bit %u flipped\n", i, j);
140 void check_double_bit_detection()
143 printf("Checking double bit detection.\n");
145 for (i = 0; i < NUM_DATA_WORDS; ++i) {
146 unsigned code = make_codeword(i);
147 for (j = 0; j < CODE_BITS; ++j) {
148 for (k = 0; k < CODE_BITS; ++k) {
149 unsigned corrupted_code = code ^ (1 << j) ^ (1 << k);
153 if (!has_error(corrupted_code)) {
154 printf("ERROR: Failed double-bit test 1 for %x with bit %u and %u flipped\n", i, j, k);
156 if (!has_double_error(corrupted_code)) {
157 printf("ERROR: Failed double-bit test 2 for %x with bit %u and %u flipped\n", i, j, k);
166 check_zero_bit_detection();
167 check_single_bit_detection();
168 check_double_bit_detection();