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Floppy Disk Controller
The floppy disk controller (FDC) contains two registers that may
be accessed by the system unit's microprocessor: a status register
and a data register. The 8-bit main status register contains the
status information of the FDC and may be accessed at any time.
The 8-bit data register (actually consisting of several registers in a
stack with only one register presented to the data bus at a time)
stores data, commands, parameters, and provides floppy disk
drive (FDD) status information. Data bytes are read from or
written to the data register in order to program or obtain results
after a particular command. The main status register can only be
read and is used to facilitate the transfer of data between the
system unit's microprocessor and FDC.
The bits in the main status register (hex 34F) are defined as
follows:
Bit
Number
Name
Symbol
Description
DBO
FDD A Busy
DAB
FDD number 0 is in the Seek mode.
DB1
FDD B Busy
DBB
FDD number 1 is in the Seek mode.
DB2
FDD C Busy
DCB
FDD number 2 is in the Seek mode.
DB3
FDD D Busy
DDB
FDD number 3 is in the Seek mode.
DB4
FDC Busy
CB
A read or write command is in process.
DB5
Non-DMA
NDM
The FDC is in the non-DMA mode.
Mode
DB6
Data Inputl
DIO
Indicates direction of data transfer
Output
between FDC and processor. If DIO
=
"1;'
then transfer is from FDC data register to
the processor. If DIO
=
"0;' then transfer
is from the processor to FDC data register.
DB7
Request for
ROM
Indicates data register is ready to send or
Master
receive data to or from the processor. Both
bits DIO and ROM should be used to
perform the handshaking functions of
"ready" and "direction" to the processor.
The
FOe
is capable of performing 15 different commands. Each
command is initiated by a multi-byte transfer from the system
unit's microprocessor, and the result after execution of the
command may also be a multi-byte transfer back to the system
4 Diskette Adapter
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