4. FDC Description (82078)
25
DIO -- Indicates the direction of a data transfer once RQM is set. A 1 indicates a read is
required and 0 indicates a write is required.
NON-DMA -- This mode is selected in the SPECIFY command and will be set to 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.
COMMAND BUSY -- This bit is set to 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
result phase. If there is no result phase (SEEK, RECALIBRATE commands), this bit is
returned to 0 after the last command byte.
DRV x BUSY -- These bits are set to 1 when a drive is in the seek portion of a
command, including implied and overlapped seeks, and recalibrates.
Data (FIFO)
3F5h Default
All command parameter information and disk data transfers go through the FIFO. The
FIFO is 16 bytes in size and has programmable threshold values. Data transfers are
governed by the RQM and DIO bits in the
Main Status Register
.
The FIFO defaults to PC/AT compatible 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 it allows the system a larger DMA latency without causing a
disk error. The two "FIFO Service Delay" tables, below, give several examples of the
delays with a FIFO. The data is based upon the following formula:
Threshold# x
1 .
DATARATE
x 8
- 1.5 µs = DELAY
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 82078
enters the command execution phase, it clears the FIFO of any data to ensure invalid
data is not transferred.
An overrun or underrun terminates the current command and the transfer of data. Disk
writes will complete the current sector by generating a 00 pattern and valid CRC. Reads
require the host to remove the remaining data so that the result phase may be entered.
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