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ELITE SERIES USER MANUAL APPEN. A - DISK I/O
“disk” bytes to store a 256 byte sector permit-
ting a full 13 sectors per track.
3. A hardware modification (the P6 PROM on
Apple’s Disk II controller card) allowed the
development of 16 sectors per track, providing a
23% increase in storage over the 13 sector for-
mat. This is possible because of a 6-plus—2
split of the data, and the PROM allowing two
consecutive zero bits in data bytes.
The storage and retrieval of information had
been restricted in 13 sector format by the re-
quirement that a disk byte have the high bit set
and, in addition, no two consecutive bits could
be zero. The odd-even 4-plus-4 technique meets
these requirements. Each data byte is repre-
sented as two bytes, containing odd and even
bits respectively. (See first figure.) Note
that the unused bits are all set to one to
guarantee meeting the two requirements.
The 4—plus—4 technique is used to store the
information (volume, track, sector, checksum)
contained in the address field. It is quite
easy to decode the data, since the byte with the
odd bits is simply shifted left and logically
ANDed with the byte containing the even bits as
shown in the second figure.
The major difficulty with the above technique is
that it occupies a lot of space on the track, so
the 5—plus—3 technique was developed, so named
because it splits the bytes into five bits plus
three bits and not in half as with the above.
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