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ELITE SERIES USER MANUAL APPEN. A - DISK I/O
A byte would have the form OOOXXXXX where X is a
valid data bit. This byte could range in value
from $00 to $1F, a total of 32 different values.
There are 34 valId “disk” bytes, ranging from
$AA to $FF, which meet the two requirements
(high bit set, no consecutive zero bits). Two
bytes, $D5 and $AA are reserved bytes thus al—
lowing an exact mapping between five bit data
bytes and eight bit “disk” bytes.
The 256 bytes that will make up a sector must be
translated to five bit bytes by a special “pre—
nibble” routine within DOS‘s RWTS which involves
bit rearrangement. The figure below, shows the
before and after of the routine. On the left is
a buffer of eight bit data bytes. Each byte in
this buffer is given a letter (A, B, C, etc) and
each bit a number (7 through 0). The right side
shows the results of the transformation. The
primary buffer contains five distinct areas of
five bit bytes (the top three bits of the eight
bit bytes zero filled) and the secondary buffer
contains three areas which gave rise to the 5-
plus—3 name.
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