ACT apricot - Page 103

FLOPPY DISK INTERFACE
and the FDC requires another byte from the lOP. The
output is reset when the Data Register is loaded with a new
byte.
The INTRQ output is activated (set to logic high) on the
successful completion of disk read, write, or formatting
operations; and automatically reset following these
operations, on reading the Status Register or issuing a new
command to the Command Register.
The INTRQ output is also set to logic high, for a variety of
other conditions (premature termination of a disk transfer
command sequence due to an error condition, completion of
a read/write head position command, etc.). In all cases,
lNTRQ can be reset by either reading the Status Register or
issuing a
command to the Command Register. These other
conditions are detailed in the following paragraphs.
The first operation performed by the FDC on receiving a
disk read or
disk write command is to check whether the
disk is ready for a transfer operation by analysing the
READY input from the disk drive. If the input is set to logic
high,
the command sequence is initiated. If the input is set
to logic low, the command sequence is immediately
aborted, the Not Ready bit set within the Status Register,
and the INTRQ output activated.
The FDC also analyses the Write Protect input (WPRT)
from the disk drive on receiving a disk write or formatting
command. If the WPRT input is activated (logic low,
indicating
that the disk is write protected), the write
operation is terminated, INTRQ is activated and the Write
Protect bit set within the Status Register.,
Prior to writing data to or reading data from the disk, the
FDC locates the correct sector for the transfer operation, by
analysing
the ID fields on the disk. Failure to detect an ID
field
with the correct track number, correct side number,
correct sector number and correct CRC within five
revolutions of the Disk, sets the Record Not Found (RNF)
bit in the Status Register, activates the INTRQ output and
terminates the transfer operation.