RANA Systems Elite Controller, User Manual

Page: 288 / 328

ELITE SERIES USER MANUAL APPEN. A - DISK I/O
A . 4 . F i e l d s
The two types of fields on a diskette are the
Address and the Data fields which both contain a
prologue, data area, checksum, and epilogue.
Each field on a track is separated from adjacent
fields by a number of bytes, called “gaps”.
All gaps are made up in the same way comprising
self-sync hexadecimal FF‘s, and vary only in the
number of bytes they contain.
The first figure below shows a portion of a
typical track divided into its major compo-
rients.
The three different types of gaps on a track are
made up of self—sync or auto—sync bytes, so—
called because they automatically bring the
hardware into sync with data bytes on the disk—
ette.
A self-sync byte is defined to be a hexadecimal
FF with a difference: it is a ten bit byte,
rather than an eight bit byte. Its two extra
bits are zeros, as shown in the second figure.
A self—sync is generated by using a 40 cycle
(slightly less than a microsecond) loop while
writing an FF. A bit is written every four
cycles, so two of the zero bits brought into the
data latch while the FF was being written are
also written to the disk, making the ten bit
byte.
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