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3-5.
SPARING
There are two sparing sequences available. One is
sector sparing and the other is track sparing. Each
track contains 63 data sectors plus one spare sec-
tor. The second sector spare operation on a track
generates a track spare operation. Each data head
has six a va
ill
a ble spare tracks. There is no cross
head sparing allowed.
If 1 or 0 spares are available after a sparing opera-
tion, the "media wear" bit (bit 51) in the CS/80
status word will be set. If a track sparing operation
is attempted with no available spares, the "No
Spare Available" bit (bit 34) is set.
3-6.
SEQUENCE
The sparing sequence is as follows:
• The host issues a Spare Block command.
• The
drivt:~
seeks to track with defective sector.
• The
drivt~
reads a sector header and looks at the
Flag Status Byte (refer to table 3-4) to deter-
mine if sector sparing has occurred.
• If sector sparing is a vaila ble, then sector sparing
is attempted.
• If sector sparing is unavailable, then track spar-
ing is attempted.
3-7.
SECTOR SPARING
A sector sparing operation is as follows:
• The disc drive performs write then read error
rate test (WTR ERT) seven times on the spare
sector using the random pattern table.
• If the WTR ERT fails the track is spared.
• If the WTR ERT, passes the track data is read
(to
Buffc;~r
0 and Buffer 1) except for the bad
sector.
Theory Of Operation
7957/7958
• Unrecoverable errors on any other sector will
cause the sparing operation to terminate and an
unrecovera ble error will be reported.
• The disc drive then writes the bad sector header
with the defective sector number 255 (FF) and
subsequent sectors are numbered sequentially
starting with the sector being spared. Data is
written to the track.
• The defective sector's data field is filled with
zeros.
3-8.
TRACK SPARING
• The disc drive looks up the next available spare
track in RAM.
• It seeks to the spare track.
• If the seek fails, the spare track is flagged as bad
in the spare track table. The sparing operation is
terminated and a Unit Fault is reported in the
CS/80 status.
• If the disc drive seeks to the correct track, a
WTR ER T is initiated seven times on sectors 0
through 62 using the pattern table.
• If WTR ERT fails, a sector sparing operation is
initiated.
• The WTR ERT is initiated again (seven times).
• If WTR ERT fails again, the disc drive looks up
the next spare track in the spare table and
repeats the previous five steps.
• If the second spare track fails, the sparing
operation is terminated and a Unit Fault is is-
sued in the CS/80 status.
• If the WTR ERT passes, data is read (to Buffer
o
and Buffer
1)
from the defective track.
• If any errors, other than the defective sector,
are detected the sparing operation fails and an
unrecovera ble error is reported.
3-5
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