ACT apricot - Data Register; Disk Drive Selection; Head Loading; Programming Considerations

FLOPPY DISK INTERFACE
location 44H in the system input/output space and can be
written to and read from. During multiple sector transfer
operations,
the sector register is updated by the FDC.
Data Register
The 8-bit Data Register is the holding register during
transfer operations to and from the disk, and is located at
address location 46H in the system input/output space. The
register performs a different function during the Seek
Command, when the register is programmed with the
desired track location.
PROGRAMMING CONSIDERATIONS
Disk Drive Selection
Selecting a disk drive for operation is achieved by writing
data to the Parallel Interface. The address location and data
required to select a specific drive is detailed in the section
on the Parallel Interface.
Disk drives are specified as Drive 0 or Drive 1 according
to the position of a switch located at the rear of each drive.
A logic
Iowan the Drive Select 0 control line selects the
disk drive configured as Drive 0, and a logic Iowan the
Drive Select 1 control line selects the disk drive configured
as Drive
1. The decoding circuitry of the in,terface is
employed
to ensure that both drives are not selected
simultaneously under software control, by only allowing
one of
the drive select lines to be set to the active logic low
state at anyone time.
Since the Track Register contains a record of the position
of the head of the drive selected, changing operation from
one drive to the second drive in dual drive systems, requires
the Track Register to be updated accordingly.
Head Loading
Loading the read/write head of the selected disk drive is
achieved by
writing data to the Parallel Interface. The
address location and data required to load the head are
detailed
in the section on the Parallel Interface.