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ELITE SERIES USER MANUAL APPEN. A - DISK I/O
The prologue comprises three bytes which form an
unique sequence found in no other component of
the track, enabling DOS to locate an address
field almost without chance of an error. The
three bytes are $D5, $AA, and $96. The first
two are reserved, thus ensuring the uniqueness
of the prologue while the $96 indicates that the
data following is an address field rather than a
data field.
The address information follows next, comprising
volume, track, and sector numbers, and a check-
sum, all of which is vital if DOS is to know
where it is positioned on a particular diskette.
The checksum is computed by exclusive-ORing the
first three pieces of information and is used to
verify its integrity.
The epilogue contains the three bytes $DE, $AA,
and $EB, the last one of which is always written
during initialization but is never verified when
an address field is read. The epilogue bytes
are often referred to as "bit-slip marks" which
provide assurance that the drive is still in
sync with the bytes on the diskette. These bytes
are not vital but are a useful double-
check. Like the address field, the data field
comprises a prologue, data, checksum, and an
epilogue as shown in following figure.
Page A-27