The MSR indicates when the disk controller is
ready to receive data via the Data Register. It
should be read before each byte transferring to or
from the data register except in DMA mode. No
delay is required when reading the MSR after a
MAIN STATUS REGISTER
Address 3F4 READ ONLY
The Main Status Register is a read-only register
and indicates the status of the disk controller. The
Main Status Register can be read at any time.
data
transfer.
7
6
5
4
3
2
1
0
NON
DMA
CMD
DRV1
DRV0
BUSY
RQM
DIO
BUSY Reserved Reserved BUSY
BIT 0 - 1 DRV x BUSY
DATA REGISTER (FIFO)
These bits are set to 1s when a drive is in the seek
portion of a command, including implied and
overlapped seeks and recalibrates.
Address 3F5 READ/WRITE
All command parameter information, disk data and
result status are transferred between the host
processor and the floppy disk controller through
the Data Register.
BIT 4 COMMAND BUSY
This bit is set to a 1 when a command is in
progress. This bit will go active after the command
byte has been accepted and goes inactive at the
end of the results phase. If there is no result
phase (Seek, Recalibrate commands), this bit is
returned to a 0 after the last command byte.
Data transfers are governed by the RQM and DIO
bits in the Main Status Register.
The Data Register defaults to FIFO disabled mode
after any form of reset. This maintains PC/AT
hardware compatibility. The default values can be
changed through the Configure command (enable
full FIFO operation with threshold control). The
advantage of the FIFO is that it allows the system
a larger DMA latency without causing a disk error.
Table 14 gives several examples of the delays
with a FIFO. The data is based upon the following
formula:
BIT 5 NON-DMA
This mode is selected in the SPECIFY command
and will be set to a 1 during the execution phase of
a command. This is for polled data transfers and
helps differentiate between the data transfer phase
and the reading of result bytes.
BIT 6 DIO
Indicates the direction of a data transfer once a
RQM is set. A 1 indicates a read and a 0 indicates
a write is required.
Threshold # x
1
x 8 - 1.5 s = DELAY
DATA RATE
At the start of a command, the FIFO action is
always disabled and command parameters must
be sent based upon the RQM and DIO bit settings.
As the command execution phase is entered, the
FIFO is cleared of any data to ensure that invalid
data is not transferred.
BIT 7 RQM
Indicates that the host can transfer data if set to a
1. No access is permitted if set to a 0.
26
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