PRETRK - Pre-Compensation Start Track Number.
Programmable from track 0 to 255. Defaults to track 0. A
"00" selects track 0; "FF" selects track 255.
Seek command is 255 (D).
The internal register, PCN, will overflow as the cylinder
number crosses track 255 and will contain 39 (D). The
resulting PCN value is thus (RCN + PCN) mod 256.
Functionally, the FDC starts counting from 0 again as the
track number goes above 255 (D). It is the user's
Version
The Version command checks to see if the controller is
an enhanced type or the older type (765A). A value of 90
H is returned as the result byte.
responsibility
to
compensate
FDC
functions
(precompensation track number) when accessing tracks
greater than 255. The FDC does not keep track that it is
working in an "extended track area" (greater than 255).
Any command issued will use the current PCN value
except for the Recalibrate command, which only looks for
the TRACK0 signal. Recalibrate will return an error if the
head is farther than 79 due to its limitation of issuing a
maximum of 80 step pulses. The user simply needs to
Relative Seek
The command is coded the same as for Seek, except for
the MSB of the first byte and the DIR bit.
DIR
Head Step Direction Control
issue a second Recalibrate command.
The Seek
DIR
ACTION
command and implied seeks will function correctly within
the 44 (D) track (299-255) area of the "extended track
area". It is the user's responsibility not to issue a new
track position that will exceed the maximum track that is
present in the extended area.
0
1
Step Head Out
Step Head In
RCN Relative Cylinder Number that determines how
many tracks to step the head in or out from the
current track number.
To return to the standard floppy range (0-255) of tracks, a
Relative Seek should be issued to cross the track 255
boundary.
The Relative Seek command differs from the Seek
command in that it steps the head the absolute number of
tracks specified in the command instead of making a
comparison against an internal register. The Seek
command is good for drives that support a maximum of
256 tracks. Relative Seeks cannot be overlapped with
other Relative Seeks. Only one Relative Seek can be
active at a time. Relative Seeks may be overlapped with
Seeks and Recalibrates. Bit 4 of Status Register 0 (EC)
will be set if Relative Seek attempts to step outward
beyond Track 0.
A Relative Seek can be used instead of the normal Seek,
but the host is required to calculate the difference
between the current head location and the new (target)
head location. This may require the host to issue a Read
ID command to ensure that the head is physically on the
track that software assumes it to be. Different FDC
commands will return different cylinder results which may
be difficult to keep track of with software without the Read
ID command.
Perpendicular Mode
As an example, assume that a floppy drive has 300
useable tracks. The host needs to read track 300 and the
head is on any track (0-255). If a Seek command is
issued, the head will stop at track 255. If a Relative Seek
command is issued, the FDC will move the head the
specified number of tracks, regardless of the internal
cylinder position register (but will increment the register).
If the head was on track 40 (d), the maximum track that
the FDC could position the head on using Relative Seek
will be 295 (D), the initial track + 255 (D). The maximum
count that the head can be moved with a single Relative
The Perpendicular Mode command should be issued
prior to executing Read/Write/Format commands that
access
a disk drive with perpendicular recording
capability. With this command, the length of the Gap2
field and VCO enable timing can be altered to
accommodate the unique requirements of these drives.
Table 31 describes the effects of the WGATE and GAP
bits for the Perpendicular Mode command. Upon a reset,
the FDC will default to the conventional mode (WGATE =
0, GAP = 0).
66
SMSC [ SMSC CORPORATION ]
