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ELITE SERIES USER MANUAL APPEN. A - DISK I/O
The data field has a checksum, similar to that
in the address field, to verify data. It also
involves exclusive-ORing data in pairs before it
is transformed by the look-up table above, best
illustrated by the figures below.
The read routine must read a byte, transform it
and store it, all in less than 32 cycles (time
to write a byte) or the information is lost. A
checksum computation to decode data greatly
speeds up the operation. As data is read from a
sector, the accumulator contains the cumulative
result of all previous bytes exclusive-ORed
together. The value of the accumulator after
any exclusive-OR is the actual data byte for
that point in the series. (Second figure.)
A third encoding technique, similar to the 5-
plus—3, still calls for the high bit to be set
but the byte may now contain one (and only one)
pair of consecutive zeros. This allows a great-
er number of valid bytes and permits the use of
a 6-plus—2 technique.
A six bit byte would have the form OOXXXXXX with
values $00 to $3F for a total of 64 different
values. With the relaxed requirements for valid
“disk” bytes, there are 69 different bytes ($96
to $FF). After removing the two reserved bytes
($AA and $D5) there are 67 “disk” bytes with
just 64 needed. With the mapping one to one
(with at least two adjacent bits set, excluding
bit 7) exactly 64 valid “disk” values are left.
The initial transformation is achieved by the
new prenibble routine with the results shown in
the last figure.
Page A-36