Table 7 Status Byte Bit Assignments - IBM 2250 3 Manual

Table 7. Status Byte Bit Assignments
Bit Name
0 Attention
Status Modifier
2 Control Unit End
3 Busy
4 Channel End
5
Device End
6 Unit Check
7
Unit Exception
32
Condition
Indicates a request for service from
alphameric keyboard or programmed
function keyboard; program should re-
spond by issuing a Read Manual Input
command. Also, the setting of both
the Attention bit and the Unit Check
bit indicates an interrupt condition,
such as light-pen detect or data check
during the regeneration cycle. The
program should respond by issuing a
Sense command.
Set with the Busy bit to indicate pend-
ing status for a 2250 other than the
addressed 2250.
Set following a Busy, Status Modifier
condition when the pending or stacked
status is cleared to indicate that the
accompanying status is the outstanding
status.
Set in response to all commands if an
interruption condition exists. The
outstanding status accompanies the busy
indication. The Busy bit is also set
with the Status Modifier bit to indicate
pending or stacked status for a 2250
which is not addressed.
Set when the transfer of data and con-
trol information between the 2840 and
the channel is complete. Both Chan-
nel End and Device End will be set for
an ending status.
Set when the 2840 has completed op-
eration on a command and is prepared
to accept a new command. Both De-
vice End and Channel End will be set
for an ending status.
Set when an unusual program or equip-
ment condition is detected at the 2840.
The program should always respond by
issuing a Sense command for further
definition of the condition.
If command execution has started,
the Channel End and Device End bits
also are set. If the condition is de-
tected during regeneration, the Atten-
tion bit is set in addition to the Unit
Check bit. Both the Attention bit and
the Unit Check bit are set to indicate
a light-pen detect condition, an End
Order Sequence order, or a Data Check.
Not used.
Channel End bits are set in response to commands
that do not cause data transfer (Set Audible Alarm,
Control No-Op, Insert Cursor, and Remove Cur-
sor). When status is pending for a device (a pre-
vious status byte is awaiting transfer to the channel)
the status byte, with the Busy bit set, is sent to
the channel in response to all commands for that
device; the command is not accepted by the 2840.
For a Test I/O instruction, the pending status byte
is presented without the Busy bit set.
A status byte is sent to the channel during an
ending operation that is at the completion of a 2840
operation involving data transfer with the channel.
The ending operation status byte always relates
to the command operation just ending. The normal
ending status byte will have only the Channel End
and Device End bits set. Any error condition as-
sociated with the operation just executed will cause
additional status bits to be set. Ending status
causes an I/
0
interrupt unless chaining is specified.
When an interrupt condition occurs while the
2840 is operating asynchronously (not selected by
the channel), the Attention bit or both the Attention
bit and the Unit Check bit will be set in the status
byte. An interrupt condition can be caused by a
light-pen detect, a programmed function key acti-
vation, an alphameric keyboard END or CANCEL
key activation, an End Order Sequence order, a
buffer parity error, etc. When an interrupt condi-
tion occurs, the 2840 requests selection from the
channel and sends the status byte to the channel
when selection is accomplished. This status byte
will be in the CSW stored by the
I/
0
interrupt
generated by the channel. The status byte is reset
by the 2840 after the status information has been
accepted by the channel.
COMMANDS
Four basic types of command are used with the
2840: Write, Read, Control, and Sense. The
Write command initiates data transfer into the
2840 buffer area for the selected 2250. Read com-
mands cause data transfer from the 2840 buffer or
from the selected 2250 registers (programmed
function keyboard, X Y position registers, etc.),
via the 2840, to the channel. Control commands
initiate the setting of programmed function indi-
cators, activate a single-stroke audible alarm at
the selected 2250 (which attracts operator attention
to the display unit), and control display regenera-
tion and cursor insertion. The Sense command
causes the transfer of sense data to the channel;
this data indicates various control and/ or check
conditions in the 2840 and a 2250.