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ELITE SERIES USER MANUAL APPEN. A - DISK I/O
The prologue differs in the third byte. The
bytes are $D5, $AA and $AD also forming an
unique sequence which enables DOS to locate the
start of the sector’s data, which consists of
342 (GCR) encoded bytes, and is followed by a
checksum byte used to verify the integrity of
the field just read. The epilogue is identical
in its function to that in the Address field.
A. 6. Data Field Encoding
Since it is not possible to read all 256 possi-
ble byte values (an Apple-GCR restriction), data
written to the diskette must be encoded. This is
done in three different ways:
1. This first method, currently used in address
fields, involves writing a data byte as two disk
bytes. One contains even bits, the other the
odd bits, thus requiring 512 “disk” bytes for
each 256 byte sector of data. Using this tech-
nique for sector data provides for no more than
10 sectors per track (about 88K of data per
diskette, or 72K of space available to the user;
typical for 5 7/4K single density drives).
Encoding the data in this way would be very
similar to the frequency modulated encoding of
data.
2. The second method allows 12 sectors per
track. It involves a 5-plus-3 split of the data
bits (as opposed to a 4—plus—4). Each byte
written to the diskette contains five actual
data bits, rather than four, thus requiring 410
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