If you have too little memory for track buffering (or for some other reason decide not
to
read a whole track
at
once), the disk hardware supports a limited set of sector-searching
facilities. There is a register
that
may be polled to examine the disk input stream.
There is a hardware bug
that
causes the last three bits of
data
sent
to
the disk to be
lost. Also, the last word
in
a disk-read DMA operation may not come in
(that
is,
one
less word may be read than you asked for).
OTHER
REGISTERS
IN
DISK
OPERATIONS
A number of other registers are also associated with disk operations, as specified below.
DSKBYTR
- Disk
Data
Byte
and
Status
Read
This register is the disk-microprocessor
data
buffer. In read mode,
data
from the disk is
loaded into this register one byte
at
a time.
As
each byte is received into the register,
the BYTEREADY bit is set true. BYTEREADY is cleared each time the DSKBYTR
register is read.
DSKBYTR is the register normally used by system software to synchronize the processor
to the disk rotation before issuing a read or write under DMA control.
The
bits are
shown in table 8-8.
232
Interface Hardware
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