Configure Default Values:
Relative Seek
EIS - No Implied Seeks
EFIFO - FIFO Disabled
POLL - Polling Enabled
The command is coded the same as for Seek,
except for the MSB of the first byte and the DIR
bit.
FIFOTHR - FIFO Threshold Set to 1 Byte
PRETRK - Pre-Compensation Set to Track 0
DIR
Head Step Direction Control
EIS - Enable Implied Seek. When set to "1", the
FDC will perform a Seek operation before
executing a read or write command. Defaults to
no implied seek.
DIR
ACTION
0
1
Step Head Out
Step Head In
EFIFO - A "1" disables the FIFO (default). This
means data transfers are asked for on a byte-
by-byte basis. Defaults to "1", FIFO disabled.
The threshold defaults to "1".
RCN Relative
Cylinder
Number
that
determines how many tracks to step the
head in or out from the current track
number.
POLL - Disable polling of the drives. Defaults to
"0", polling enabled. When enabled, a single
interrupt is generated after a reset. No polling is
performed while the drive head is loaded and
the head unload delay has not expired.
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
FIFOTHR - The FIFO threshold in the execution
phase of read or write commands. This is
programmable from 1 to 16 bytes. Defaults to
one byte. A "00" selects one byte; "0F" selects
16 bytes.
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.
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.
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
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.
65
SMSC [ SMSC CORPORATION ]
