As can be seen from the examples, there are gaps containing zero and/or 4EH
bytes between the ID blocks and data areas, before and after the index mark (if
present) and at the end of the track. These gaps are there to allow the FDC to
synchronize with the disk to read the data and ID blocks.
The gaps before the index mark and at the end of the track are known as Gap 4.
The gap after the index mark and before the first ID block is known as Gap 1.
Gap 2 separates each ID block from its associated data area while Gap 3
separates the data area from the next ID block on the disk.
The values of the bytes used to create the gaps are different for single density
and double density modes as are the number of bytes used. Below is an extract
from the manufacturers data sheet for the FD1797 showing the byte values and
byte counts for the gaps. The values shown for the byte counts are minimum
except where shown.
The physical data is held on the disk surface as a serial data stream. In double
density mode a 250ns pulse is sent to the disk drive for each flux transition.
This would imply that a pulse is sent to the drive each time the data changed
from 0 to 1 or 1 to 0. In addition to the data, a clock is recorded on the disk
surface. This clock is picked off when reading the disk to synchronize the date
being read. The presence of a clock on the disk is used to good effect by the
FD1797 when sending control bytes to the disk.
An in the example track for byte values of 0F5H and 0F6H generate the
values 0A1H and 0C2H on the disk surface. However, so that these bytes can be
distinguished from data bytes of the same values, clock pulses are deliberately