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Functional Description
7936 and 7937
track in this manner, an additional latency is not
incurred when accessing a track containing a
spared sector.
When reformatting the target track, the controller
writes the physical address of the bad sector in the
header of each sector on the track. Using this in-
formation, the controller knows which sector to
skip in subsequent accesses to the track. On tracks
that have not undergone a sparing operation, the
field in the sector headers contains the physical
address of the spare sector (logical sector 123).
5-10.
TRACK SPARING
If the spare sector has been used, controller
PCA-A6 must copy the entire track on which the
defective sector resides to an available spare track.
Each data head has 6 spare tracks allocated for
this purpose. Like data tracks, these spare tracks
are organized into cylinders and, to improve spar-
ing efficiency, are located in two bands on each
data surface. See figure 5-1. A spare track table,
which is stored in controller RAM, is generated at
power on by reading the
10
and
00
spare pools
sequentially until a track is found that has not
been spared out. This table is then searched before
a seek begins to determine if a particular logical
address has been reallocated and if so the address
is pulled from the table. To reduce the amount of
controller RAM consumed by the spare track
table, scalar values, rather than explicit addresses,
are used to record the physical location of each
spare track. Figure 5-5 shows the relationship of
scalar values to physical spare track locations.
When performing a track sparing operation, the
controller first determines the nearest (in terms of
seek time) from the spare pool. Then the next
sequential scalar in the pool is the track that is
used in the spare. If the track to be spared resides
at logical cylinder 0-697, then the
00
spare pool is
5-8
used (scalar number 39-77). If the track is at
cylinder 698-1395, the the
10
spare pool is used
(scalar number 0-38).
If all of the spare tracks are used up in one of the
spare pools, then the next sequential scalar from
the opposite spare pool will be chosen.
Defective
maintenance
tracks
are
spared
automatically by controller PCA-A6. No host
intervention
is
required
when
sparing
a
maintenance track.
5-11.
CONTROLLER PCA-AS
Controller PCA-A6 provides an interface between
the host computer and the drive electronics. One
of two available HP-IB controllers is factory instal-
led in the drive mainframe.
Appendix A at the rear of this manual provides
details of the controllers, including a functional
block diagram and accompanying circuit descrip-
tion. Refer to Appendix A for details of the con-
troller installed in the drive.
NOTE
In this manual, the controllers are refer-
red to collectively as "PCA-A6".
Each controller includes: host interface, a data
buffer for data examination and speed matching,
disc memory access (DMA) for control of data
transfer, error correction (ECC), firmware in read
only memory (ROM) for an on-board microproces-
sor to execute drive commands and monitor status,
a peripheral interface for command and status in-
terface with the drive electronics, and channel ad-
dress/diagnostic control switches. See figure 5-16.
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