Servo-Tracking Faults; Data Compression; Background - Seagate STU62001LW Viper Product Manual

Theory of operations
C1 ECC
C2 ECC

Servo-tracking faults

Data compression

Background

As data is written to memory from the Data Processing unit, the DMA / ECC interface
generates C1 ECC bytes and writes them to memory.
As data is written to tape, the C1 ECC is checked, and an interrupt generated if there
is an error. The C1 ECC read from memory is the ECC that is written to tape.
When data is read from tape and stored into memory, C1 ECC is checked. If the C1
ECC is good, then that codeword pair's "Valid" bit is set. Otherwise, a pointer to the
invalid Codeword Pair is passed to the C1 ECC correction engine. If it can correct
the error, then the corrected bytes are written to memory, and the Valid bit is set.
Otherwise, the Valid bit is left cleared. As data is read from memory to the Data
Processor for de-compression, the C1 ECC is again checked, and an interrupt
generated if it is not correct.
C2 ECC involves three distinct operations:
1. Encoding: Generating C2 ECC bytes from data bytes (performed by ECC
co-processor hardware)
2. Decoding: Generating ECC syndromes from data and ECC bytes, testing
for all-zeroes (performed by ECC co-processor hardware)
3. Correction: Generating corrected data from syndromes.
Correction is performed differently depending on the number and types of errors
involved:
•
For one known C1 codeword pair in error in a sub-data set (C2 codeword)
the operation is performed by ECC co-processor hardware.
•
For two or more known C1 codeword pairs in error, the matrix is computed
by firmware and the correction is performed by hardware.
•
For One or more unknown C1 codeword pairs, syndromes are generated by
hardware, error location is computed by firmware, the matrix is computed by
firmware, and correction performed by hardware.
If, while performing a write operation, the servo system detects an error that may
result in adjacent data tracks being over-written; the write operation will be aborted.
The write operation will not continue until the correct servo tracking is re-established.
Typical data streams of text, graphics, software code, or other forms of data contain
repeated information of some sort, whether it is at the text level where you can
readily recognize regular repetitions of a single word, or at the binary level where the
repetitions are in bits or bytes. Although most data is unique and random, the binary
37