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Introduction
All operations performed by the unit are related
to
data storage and recovery
(normally referred to as writing and reading). The actual reading is performed by
U4~
electromagnetic devices called heads that are positioned over the recording sur-
faces of the rotating discs. There are two heads for each disc surface on the 413M-
byte unit and one head for each disc surface on the 132M-byte unit. The heads are
positioned in such a way that data is written in concentric tracks around the disc
surfaces.
Before any read or write operation can be performed, the controller must instruct
the unit to position the heads over the desired track (seek) and also to use the
head located over the surface (head selection) where the operation is to be
performed.
After selecting a head and arriving at the data track, the controller still must
locate that portion of the track on which the data is to be written or read. This
action (track orientation) is done by using the index and sector signals generated
by the unit. The index signal indicates the logical beginning of each track, and
the sector signals are used by the controller to determine the position of the head
on the track with respect to index.
When the desired location is reached, the controller commands the unit to read or
write the data. During a read operation, the unit recovers data from the discs and
transmits it to the controller. During a write operation, the unit receives data
from the controller, processes it and writes it on the discs.
The unit is also capable of recognizing certain errors that may occur during its
operation. When an error is detected, it is indicated either by a signal to the con-
troller or by a maintenance indicator on the uriit itself.
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HD42-OO
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