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The data gap, servo sync, and servo code fields
which follow the postamble comprise the sampled
(embedded) servo data. This code is described in
more detail in the description of the servo system
electronics.
5-6.
ADDRESSING STRUCTURE
The head-disc assembly in the HP 7936 contains
seven data surfaces, with the data being accessed
with seven read/write heads. See figure 5-4. The
HDA in the HP 7937 contains 13 data surfaces,
with data being accessed with 13 read/write heads.
Head positioning information and sector clocking
are derived from the dedicated servo surface
through the read-only servo head. There are 1396
ensured cylinder positions available for data
storage (HP 7936 and HP 7937). Cylinder addresses
range from 0 to 1395. Each cylinder consists of
seven (HP 7936) or 13 (HP 7937) data tracks, one
for each data surface. Tracks are addressed when
both cylinder and head addresses are specified.
Each data track is divided into 123 logically ad-
dressable physical sectors (HP 7936 and HP 7937).
Sectors are addressed when both head and sector
addresses are specified for a given cylinder. Head
addresses range from zero to 6 (HP 7936) or zero
to 12 (HP 7937).
All addressing in the drive is logical. Controller
PCA-A6 assigns logical addresses to the physical
addresses. Figure 5-1 shows logical versus physical
track addressing. When a defective physical track
is encountered, a new physical track (spare track)
will be assigned to the same logical address. This
eliminates dual seeks to obtain the the correct data
and reduces system overhead in managing the discs
and spare tracks.
A total of 1396 tracks are
guaranteed as logical tracks through the the use of
spares, which the controller assigns as required.
There are 124 physical sectors. Of these, 123 sec-
tors (addressed from 0 to 122) are available for
da ta storage and one sector is reserved for use as a
spare in the event that one of the original 123 be-
comes defective. Sectors are spared by the con-
troller. In the event that the spare sector is already
used, the entire logical track will then be assigned a
new physical address (one of the spare tracks).
This sparing action is transparent to the host CPU,
Functional Description
7936 and 7937
except that the host may note the decision to
spare, and issue the spare command.
5-7.
ADDRESSING MODE
The drive operates in a cylinder mode to access
the data storage areas of the drive. In the cylinder
mode, the heads are positioned over a particular
cylinder and then data is written or read, starting
at the lowest numbered head and continuing to the
highest numbered head. A cylinder of information
consists of all sectors on all tracks at a given
cylinder address. Head switching occurs after the
data in sector 123 of the current track has been
transferred. Head switching is sequential, that is,
head I will be selected after head 0, and so on.
Data transfers will continue with sector 0 of the
next track after the address fields and track status
indicators of a sector of that track have been
verified by the drive.
5-8.
DISC SPARING
One sector per track and six tracks per data sur-
face (head) are allocated for sparing bad sectors
discovered during data transfers. When perform-
ing a sparing operation, controller PCA-A6 at-
tempts a sector spare on the target address (sector).
If the allocated spare sector has been used, the en-
tire data track is spared. When sparing, the host has
the option of retaining or not retaining data. If the
host chooses not to retain data, all data on the tar-
get track is destroyed. If the host chooses to retain
data, all data on the target track, excluding the sec-
tor being spared, is kept.
5-9.
SECTOR SPARING
When performing a sector sparing operation that
retains data, controller PCA-A6 copies the entire
contents of the target track (excluding the defec-
tive sector) to the nearest available spare track for
temporary storage. The controller then reformats
the original track, placing the spare sector into user
accessible area and mapping out the bad sector.
The data is then copied from the spare track back
to the reformatted track with the bad sector being
skipped. This shifts all those sectors with logical
addresses greater than the bad sector to the next
higher sector address. By reformatting the entire
5-7
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