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Disc Interleave
Interleaving a disc causes the sectors on the disc to be numbered according to a
specific interval. For instance, an interleave factor of 1 causes sectors to be numbered
consecutively. A factor of 2, on the other hand, causes the system to skip every other
sector. The following drawing illustrates these two interleave factors.
Figure 3-9. Examples of Disc Interleave
The BASIC system numbers each sector on the disc using the same interleave factor;
that is, each file cannot have a separate interleave factor.
The purpose of disc interleave is to increase data-transfer rates, as demonstrated in the
following example. Suppose that we are entering data from a spinning disc; suppose
also that the data have formats which are different from the computer's internal data
formats. Consequently, after each item is read, the computer must change the data from
the disc's format to the computer's internal format. And in the meantime, the disc is
still spinning.
If the processing of all items in a sector takes more time than it takes for the next sector
to pass under the read/write head, then the system must wait one full revolution of the
disc until the next sector again passes under the head. By interleaving a disc, you can
provide time for this processing, thus not making the system wait as long until the next
sector passes under the read/write head.
Mass Storage Concepts
3-19
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