Advertisement
For most devices, the first six bits of the sense
data describe situations detected during the last
operation. These bits are common to all devices
having this type of information and are designated
as follows:
Bit
Designation
0
Command reject
1
Intervention required
2
Bus-out check
3
Equipment check
4
Data check
5
Overrun
The following is the meaning of the first six bits:
Command Reject: The device has detected a
programming error. A command has been received
which the device is not designed to execute, such as
read backward issued to a direct-access storage
device, or which the device cannot execute because
of its present state, such as write issued to a
file-protected tape unit. Command reject is
indicated when the program issues an invalid
sequence of commands, such as write to a
direct-access storage device without previous
designation of the block. Command reject may
also be indicated when invalid data is transferred
and the data is treated as an extension of the
command. For example, command reject is
indicated when an invalid seek argument is
transferred to a direct-access storage device.
Intervention Required: The last operation could
not be executed because of a situation requiring
some type of intervention at the device. This bit
indicates situations such as the hopper in a card
punch being empty or the printer being out of
paper. It is also turned on when the addressed
device is not ready, is in test mode, or is not
provided on the control unit.
Bus-Out Check: The device or the control unit
has received a data byte or a command code with
an invalid parity'from the channel. During writing,
bus-out check indicates that incorrect data has
been recorded at the device, but this does not cause
the operation to be terminated prematurely. Parity
errors on command codes and control information
cause the operation to be immediately terminated
and suppress checking for situations that would
cause command reject and intervention required.
12-38
IBM 4300 Processors Principles of Operation
Equipment Check: During the last operation, the
device or the control unit has detected equipment
malfunctioning, such as an invalid card -hole count
or a printer-buffer parity error.
Data Check: The device or the control unit has
detected a data error other than those included in ,
bus-out check. Data check identifies errors
associated with the recording medium and includes
errors such as reading an invalid card code or
detecting invalid parity on data recorded on
magnetic ta'pe.
'
On an input operation, data check indicates that
incorrect data may have been placed in storage.
The control unit forces correct parity on data sent
to the channel. On writing, data check indicates
that incorrect data may have been recorded at the
device. Unless the operation is of a type where the
error precludes meaningful continuation, data
errors on reading and writing do not cause the'
operation to be terminated prematurely.
Overrun: The channel has failed to respond on
time to a request for service from the device.
Overrun can occur when data is transferred to or
from a non buffered control unit operating with a
synchronous medium, and the total activity initiated
by the program exceeds the capability of the
channel. When the channel fails to accept a byte
on an input operation, the following data
transferred to storage may be used to fill the gap.
On an output operation, overrun indicates that data
recorded at the device may be invalid. The overrun
bit is also set to one when the device receives the
new command too late during command chaining.
All information significant to the use of the
device normally is provided in the first two bytes.
Any bit positions following those used for
programming information contain diagnostic
information, which may extend to as many bytes as
needed. The amount and the meaning of the status
information are peculiar to the type of I/O device
and are specified in the SL publication for the
device.
The basic sense command has zero modifier bits.
This command initiates a sense operation on all
devices and cannot cause the command-reject,
intervention-required, data-check, or overrun bit to
be set to one. If the control unit detects an
equipment malfunction, or invalid parity of the
sense command code, the equipment-check or bus-
out-check bit is set to one, and unit check is
indicated in the unit-status byte.
Devices that can provide special diagnostic sense
information or can be instructed to perform other
Advertisement
Chapters
