above 255 (D). It is the user's 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 issue
Relative Seek
The command is coded the same as for Seek,
except for the MSB of the first byte and the DIR
bit.
DIR
0
1
ACTION
Step Head Out
Step Head In
a second Recalibrate command.
The Seek
DIR
Head Step Direction Control
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.
RCN Relative Cylinder Number that determines
how many tracks to step the head in or out
from the current track number.
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
To return to the standard floppy range (0-255) of
tracks, a Relative Seek should be issued to cross
the track 255 boundary.
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
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.
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 Seek command is 255 (D).
The Perpendicular Mode command should be
issued prior to executing Read/Write/Format
commands that access
perpendicular recording capability.
a
disk drive with
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 28
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).
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
Selection of the 500 Kbps and
1
Mbps
perpendicular modes is independent of the actual
65
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