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Configure Default Values:
EIS - No Implied Seeks
EFIFO - FIFO Disabled
POLL - Polling Enabled
FIFOTHR - FIFO Threshold Set to 1 Byte
PRETRK - Pre-Compensation Set to Track 0
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.
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".
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.
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.
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.
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.
Relative Seek
The command is coded the same as for Seek, except for the MSB of the first byte and the DIR bit.
DIR
RCN
Head Step Direction Control
Relative Cylinder Number that determines how many tracks to step the head in or out from
the current track number.
DIR
0
1
ACTION
Step Head Out
Step Head In
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.