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To facilitate system applications where a consistent number of
sectors are required per cylinder and to allow reformatting of
single cylinders when required by field grown defects,
the ACB-
5500
allows a specified number of sectors per cylinder to be
spared during formatting.
The number of spare sectors allocated
to
each
cylinder
is
variable
from
0
to
N-l
(N
=
#
of
sectors/track) and is specified in the FORMAT UNIT format infor-
mation.
For every spare sector allocated, one less sector will
be available on each cylinder.
To assure defects within a
cylinder
do
not
cause sectors
to
be shifted
into
the
next
cylinders,
a number of spare sectors greater than the expected
number of defects per cylinder should be chosen.
Figure 6-7
shows the effect of sector sparing.
CYLINDER
2
198199Il0~1· • • • • • 119411951 SIS
I
S
I
No Defects;
3 Spare Sectors
._-------------------------------------------
CYLINDER
2
119411951 S
I
2 Defects;
3 Spare Sectors
Figure 6-7.
Sector Sparing within Cylinders
If the number of defects within a cylinder should exceed the
number of spare sectors, the additional sectors will overflow
into the next cylinder.
Assuming the next cylinder has enough
spare sectors to account for defects within the cylinder and the
overflow sectnrs, no other cylinders will be affected.
This
cylinder will now contain all of its assigned sectors plus the
overflow.
Figure 6-8 describes the effects of overflow sectors.
500504-00
6-13
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