CRC-64#
This operation computes an arbitrary CRC up to 64-bytes in size.
The polynomial is expressed in the normal or “not bit-reversed” form in
the job’s qpl_job.crc64_poly field. The most-significant bit of the polynomial
(which is always 1) is not expressed, and the next most-significant bit
is written to bit-63 of the field. That is, the polynomial is written to
the high end of the field. If the polynomial is for a 64-bit CRC, then
bit-0 would also be set. If the CRC is smaller, then the
least-significant bit set will be at a higher bit position.
For example, the CRC32 used by DEFLATE is defined by the 33-bit
polynomial 104c11db7.
To generate this CRC with the qpl_operation.qpl_op_crc64 operation,
one would set the qpl_job.crc64_poly field to: 04c11db700000000. Since the
least-significant set bit is bit-32, then the size of the resulting CRC
would be 64-32 or 32.
By default, the data is viewed as Little Endian. If the
QPL_FLAG_CRC64_BE flag is used, the data is viewed as Big Endian.
When the data is LE, bit-0 of each byte is the least significant.
Additionally, bit-0 of byte-0 is the least significant bit of the
output. If the polynomial is smaller than 64-bits, the CRC is found in
the high end of the qpl_job.crc64 field.
When the data is BE, bit-0 of each byte is the most significant. Bit-7
of byte-7 of the result is then the least significant bit. If the
polynomial is smaller than 64-bits, the CRC is found in the low end of
the qpl_job.crc64 field.
This operation may not be linked, so the initial CRC is logically always
0, but many CRC invert (in a bitwise sense) the initial CRC and the
final CRC. To achieve this, the application should use the
QPL_FLAG_CRC64_INV flag.
Note
The number of bits to be inverted is implied by the size of the polynomial (i.e. determined by where the least-significant set bit is found).
Here is some well-known CRC’s, with corresponding settings and place to find the result:
CRC |
crc64_poly |
Bit Order |
Invert CRC |
crc64 (Output) |
|---|---|---|---|---|
CRC32 (gzip) |
04C11D B700000000 |
|
||
CRC32 (wimax) |
04C11D B700000000 |
XXXXXXX X00000000 |
||
T10DIF |
8BB700 0000000000 |
|
||
CRC- 16-CCITT |
102100 0000000000 |
000000000 000XXXX |