Execution Phase
For simplicity, command handling in the FDC can
be divided into three phases: Command,
Execution, and Result. Each phase is described in
the following sections.
All data transfers to or from the FDC occur during
the execution phase, which can proceed in DMA
or non-DMA mode as indicated in the Specify
command.
Command Phase
After a reset, the FIFO is disabled. Each data byte
is transferred by an FINT or FDRQ depending on
the DMA mode. The Configure command can
enable the FIFO and set the FIFO threshold value.
After a reset, the FDC enters the command phase
and is ready to accept a command from the host.
For each of the commands, a defined set of
command code bytes and parameter bytes has to
be written to the FDC before the command phase
is complete. (Please refer to TABLE 20 for the
command set descriptions). These bytes of data
must be transferred in the order prescribed.
The following paragraphs detail the operation of
the FIFO flow control. In these descriptions,
<threshold> is defined as the number of bytes
available to the FDC when service is requested
from the host and ranges from 1 to 16. The
parameter FIFOTHR, which the user programs, is
one less and ranges from 0 to 15.
Before writing to the FDC, the host must examine
the RQM and DIO bits of the Main Status Register.
RQM and DIO must be equal to "1" and "0"
respectively before command bytes may be
written. RQM is set false by the FDC after each
write cycle until the received byte is processed.
The FDC asserts RQM again to request each
parameter byte of the command unless an illegal
command condition is detected. After the last
parameter byte is received, RQM remains "0" and
the FDC automatically enters the next phase as
defined by the command definition.
A low threshold value (i.e. 2) results in longer
periods of time between service requests, but
requires faster servicing of the request for both
read and write cases. The host reads (writes)
from (to) the FIFO until empty (full), then the
transfer request goes inactive. The host must be
very responsive to the service request. This is the
desired case for use with a "fast" system.
A high value of threshold (i.e. 12) is used with a
"sluggish" system by affording a long latency
period after a service request, but results in more
frequent service requests.
The FIFO is disabled during the command phase
to provide for the proper handling of the "Invalid
Command" condition.
Non-DMA Mode - Transfers from the FIFO to the
Host
The FINT pin and RQM bits in the Main Status
Register are activated when the FIFO contains
(16-<threshold>) bytes or the last bytes of a full
sector have been placed in the FIFO. The FINT
pin can be used for interrupt-driven systems, and
RQM can be used for polled systems. The host
must respond to the request by reading data from
the FIFO. This process is repeated until the last
byte is transferred out of the FIFO. The FDC will
deactivate the FINT pin and RQM bit when the
FIFO becomes empty.
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