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 x)
21 * This variant seems to be slightly faster, but depends on
22 * fast hardware multiplication.
25 x = (x ^ (x >> 2)) & 0x11111111;
33 return (0x6996 >> x) & 1;
37 unsigned generate_parity(unsigned data)
39 unsigned parity1 = find_parity_32(data & 0x036ad555);
40 unsigned parity2 = find_parity_32(data & 0x02d9b333);
41 unsigned parity3 = find_parity_32(data & 0x01c78f0f);
42 unsigned parity4 = find_parity_32(data & 0x003f80ff);
43 unsigned parity5 = find_parity_32(data & 0x00007fff);
44 unsigned parity6 = find_parity_32(data & 0x03b4e996);
46 return parity6 | (parity5 << 1) | (parity4 << 2) | (parity3 << 3) | (parity2 << 4) | (parity1 << 5);
49 unsigned make_codeword(unsigned data)
51 return (data << PARITY_BITS) | generate_parity(data);
54 /* can detect all single or double bit errors */
55 int has_error(unsigned code)
57 unsigned parity = code & ((1 << PARITY_BITS) - 1);
58 return (generate_parity(code >> PARITY_BITS) != parity);
61 int has_double_error(unsigned code)
63 unsigned parity_diff = generate_parity(code >> PARITY_BITS) ^ code;
64 return (parity_diff & ((1 << PARITY_BITS) - 1)) && !find_parity_32(code);
67 /* Correct any single-bit error -- assumes there are no double-bit errors */
68 unsigned correct_single_bit_error(unsigned code)
70 unsigned parity_diff = generate_parity(code >> PARITY_BITS) ^ code;
73 for (i = 0; i < PARITY_BITS - 1; ++i) {
74 if (parity_diff & (1 << (PARITY_BITS - 1 - i))) {
80 /* flip the wrong bit */
81 code ^= (1 << permutation_table[bp]);
84 /* recompute the lower parity */
85 return (code & ~1) | find_parity_32(code & ~1);
88 void check_zero_bit_detection()
91 printf("Checking zero bit detection.");
94 for (i = 0; i < NUM_DATA_WORDS; ++i) {
95 unsigned code = make_codeword(i);
97 if ((i & 0xfffff) == 0) {
102 if (has_error(code)) {
103 printf("ERROR: Failed zero-bit test 1 for %x\n", i);
105 if (has_double_error(code)) {
106 printf("ERROR: Failed zero-bit test 2 for %x\n", i);
113 void check_single_bit_detection()
116 printf("Checking single bit detection and correction.");
119 for (i = 0; i < NUM_DATA_WORDS; ++i) {
120 unsigned code = make_codeword(i);
122 if ((i & 0xfffff) == 0) {
127 for (j = 0; j < CODE_BITS; ++j) {
128 unsigned corrupted_code = code ^ (1 << j);
130 if (!has_error(corrupted_code)) {
131 printf("ERROR: Failed single-bit test 1 for %x with bit %u flipped\n", i, j);
133 if (has_double_error(corrupted_code)) {
134 printf("ERROR: Failed single-bit test 2 for %x with bit %u flipped\n", i, j);
136 if (correct_single_bit_error(corrupted_code) != code) {
137 printf("ERROR: Failed single-bit correction test for %x with bit %u flipped\n", i, j);
145 void check_double_bit_detection()
148 printf("Checking double bit detection.");
151 for (i = 0; i < NUM_DATA_WORDS; ++i) {
152 unsigned code = make_codeword(i);
154 if ((i & 0xfffff) == 0) {
159 for (j = 0; j < CODE_BITS; ++j) {
160 for (k = 0; k < CODE_BITS; ++k) {
161 unsigned corrupted_code = code ^ (1 << j) ^ (1 << k);
165 if (!has_error(corrupted_code)) {
166 printf("ERROR: Failed double-bit test 1 for %x with bit %u and %u flipped\n", i, j, k);
168 if (!has_double_error(corrupted_code)) {
169 printf("ERROR: Failed double-bit test 2 for %x with bit %u and %u flipped\n", i, j, k);
180 check_zero_bit_detection();
181 check_single_bit_detection();
182 check_double_bit_detection();