#define DATA_BITS 11
#define PARITY_BITS 5
+#define EXTRA_BIT_POSITION (PARITY_BITS - 1)
#define CODE_BITS (DATA_BITS + PARITY_BITS)
#define NUM_DATA_WORDS (1 << DATA_BITS)
unsigned char hamming_lookup[NUM_DATA_WORDS];
+/*
+ * Needed since we store all the parity at the end of the word, not at the expected
+ * power-of-two bit positions. This is the inverse of the mapping
+ * (0..15) -> (0, 8, 4, 2, 1, the rest in ascending order)
+ */
+unsigned char permutation_table[CODE_BITS] = {
+ 0, 4, 3, 5, 2, 6, 7, 8, 1, 9, 10, 11, 12, 13, 14, 15
+};
+
unsigned generate_parity(unsigned data)
{
unsigned bits[DATA_BITS];
unsigned parity[PARITY_BITS];
unsigned i;
- parity[4] = 0;
+ parity[EXTRA_BIT_POSITION] = 0;
for (i = 0; i < DATA_BITS; ++i) {
bits[i] = (data & (1 << i)) ? 1 : 0;
- parity[4] ^= bits[i];
+ parity[EXTRA_BIT_POSITION] ^= bits[i];
}
parity[0] = bits[0] ^ bits[1] ^ bits[3] ^ bits[4] ^ bits[6] ^ bits[8] ^ bits[10];
parity[1] = bits[0] ^ bits[2] ^ bits[3] ^ bits[5] ^ bits[6] ^ bits[9] ^ bits[10];
parity[2] = bits[1] ^ bits[2] ^ bits[3] ^ bits[7] ^ bits[8] ^ bits[9] ^ bits[10];
parity[3] = bits[4] ^ bits[5] ^ bits[6] ^ bits[7] ^ bits[8] ^ bits[9] ^ bits[10];
- parity[4] ^= parity[0] ^ parity[1] ^ parity[2] ^ parity[3];
+ parity[EXTRA_BIT_POSITION] ^= parity[0] ^ parity[1] ^ parity[2] ^ parity[3];
- return parity[4] | (parity[3] << 1) | (parity[2] << 2) | (parity[1] << 3) | (parity[0] << 4);
+ return parity[EXTRA_BIT_POSITION] | (parity[3] << 1) | (parity[2] << 2) | (parity[1] << 3) | (parity[0] << 4);
}
unsigned make_codeword(unsigned data)
return 1;
}
+/* Correct any single-bit error -- assumes there are no double-bit errors */
+unsigned correct_single_bit_error(unsigned code)
+{
+ unsigned bits[CODE_BITS];
+ unsigned parity[PARITY_BITS];
+ unsigned i, bp = 0;
+
+ parity[EXTRA_BIT_POSITION] = 0;
+
+ for (i = 0; i < CODE_BITS; ++i) {
+ bits[i] = (code & (1 << i)) ? 1 : 0;
+ }
+ for (i = 1; i < CODE_BITS; ++i) {
+ parity[EXTRA_BIT_POSITION] ^= bits[i];
+ }
+
+ parity[0] = bits[PARITY_BITS+0] ^ bits[PARITY_BITS+1] ^ bits[PARITY_BITS+3] ^ bits[PARITY_BITS+4] ^ bits[PARITY_BITS+6] ^ bits[PARITY_BITS+8] ^ bits[PARITY_BITS+10];
+ parity[1] = bits[PARITY_BITS+0] ^ bits[PARITY_BITS+2] ^ bits[PARITY_BITS+3] ^ bits[PARITY_BITS+5] ^ bits[PARITY_BITS+6] ^ bits[PARITY_BITS+9] ^ bits[PARITY_BITS+10];
+ parity[2] = bits[PARITY_BITS+1] ^ bits[PARITY_BITS+2] ^ bits[PARITY_BITS+3] ^ bits[PARITY_BITS+7] ^ bits[PARITY_BITS+8] ^ bits[PARITY_BITS+9] ^ bits[PARITY_BITS+10];
+ parity[3] = bits[PARITY_BITS+4] ^ bits[PARITY_BITS+5] ^ bits[PARITY_BITS+6] ^ bits[PARITY_BITS+7] ^ bits[PARITY_BITS+8] ^ bits[PARITY_BITS+9] ^ bits[PARITY_BITS+10];
+
+ for (i = 0; i < PARITY_BITS - 1; ++i) {
+ if (parity[i] != bits[PARITY_BITS - 1 - i]) {
+ bp |= (1 << i);
+ }
+ }
+
+ if (bp != 0) {
+ /* flip the wrong bit */
+ code ^= (1 << permutation_table[bp]);
+ parity[EXTRA_BIT_POSITION] ^= 1;
+ }
+
+ /* recompute the lower parity */
+ return (code & ~1) | parity[EXTRA_BIT_POSITION];
+}
+
void check_zero_bit_detection()
{
unsigned i;
void check_single_bit_detection()
{
unsigned i, j;
- printf("Checking single bit detection.\n");
+ printf("Checking single bit detection and correction.\n");
for (i = 0; i < NUM_DATA_WORDS; ++i) {
unsigned code = make_codeword(i);
if (has_double_error(corrupted_code)) {
printf("ERROR: Failed single-bit test 2 for %x with bit %u flipped\n", i, j);
}
+ if (correct_single_bit_error(corrupted_code) != code) {
+ printf("ERROR: Failed single-bit correction test for %x with bit %u flipped\n", i, j);
+ }
}
}
}