ACT apricot - Track Format

FLOPPY DISK INTERFACE
TRACK FORMAT
Data is recorded on a disk in concentric circles, known as
tracks.
When a disk is inserted into the disk drive, the Auto
Shutter is opened to allow the read/write head of the disk
drive access to the disk surface. When the head is loaded,
the head makes physical contact with the radial slot of
magnetic material exposed, when the shutter is open.
Information on a track is read and written serially as the
disk rotates within its protective plastic shell.
Each track of the disk is divided into nine sectors by
software formatting (soft sectoring). The sectors are
recorded
onto each track of the disk by issuing a format
command to the FDC and then writing all the bytes onto
the disk as illustrated in Figure 2. The start of each track is
marked by a single index pulse, which is generated by the
disk drive every revolution of the disk.
Each
sector has an identification (ID) field and a data field,
separated by gaps of unintelligent information. The gaps are
required
to allow the FDC to process information from the
disk during disk reads, and also to take into account
variations in drives, such as motor speed variations. The ID
field defines the data field that follows, by specifying its
track number, side number, sector number and length of the
data field in bytes.
Both
the ID and data fields begin with an address mark
and end with a two-byte cyclic redundancy check (CRC)
character for detecting errors in the previous field. Different
address marks are used to distinguish between the two
fields.
All the serial data on the track (ID fields, data fields and
gap information) are recorded on the disk by Modified
Frequency
Modulation (MFM). In MFM encoding, both
serial data bits and clock pulses are interleaved into the data
stream.
In order to distinguish the address marks on the disk from
data bytes which may be identical, the address marks are
recorded
with missing clock pulses in predefined locations.