HP C2233 Product Description Manual page 47

Error Correction Code
DRAFT
10/24/90 11:07
The HP C2233/34/35 Series of disk drives use a
Reed-Solomon error correction code (ECC) for detection
and correction of data errors. During a write operation, .
the ECC function generates 18 bytes of ECC information,
and writes the information into the ECC field as the
sector is written. During a read operation, the controller
generates an l8-byte code from the data field being read,
and compares it to the ECC field created during the write
operation. If the l8-byte code differs from the ECC field, a
data error is detected and the ECC field is used to correct
the data.
The ECC function is enabled or disabled via the DCR
(Disable Correction) bit in parameter page Olh of the
MODE SELECT command. When enabled, the ECC
algorithm divides a sector's data field into three interleaves,
or rows, with a selectable correction factor of one, two, or
three bytes per interleave. Mathematically, this converts
to a maximum burst size of 72 bits per sector. However,
the maximum number of bits that is guaranteed to fit into
nine contiguous bytes is 65. Therefore, if an error burst
longer than 65 bits falls exactly within the boundaries of
nine contiguous bytes, it will be corrected. If it spreads
across more than nine contiguous bytes, it will be flagged as
unrecoverable.
The number of bytes that will be corrected in an interleave
is selectable via the Correction Span field in parameter
page Olh of the MODE SELECT command. The Correction
Span field value is stated in bits-per-sector. The ECC
algorithm converts this value to bytes-per-interleave by
dividing it by 24 (8-bits per byte times three interleaves)
and rounding it up to the nearest byte value.
Cyclic Redundancy Check (CRC)
ECC is aided by a 2-byte cyclic redundancy check (CRC)
to decrease the probability of error miscorrection. With
the correction span set to 72 bits, if a random error
Product Features 3-7