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1/0 Instructions
Test 1/0
Halt 1/0
In addition to initiating one of the
1/0
operations by means of the Start
1/0
instruction, the program can cause certain actions at the tape control by using the
Test
1/0
and Halt
1/0
instructions.
This instruction performed by the Central Processing Unit
(CPU),
causes the
status byte for the selected tape unit to be sent to the channel for analysis. No
actual operation is performed.
Note: A Test
1/0
command issued to a not ready tape unit results in a contingent
connection on tape controls with the Two-Channel Switch.
This instruction causes data transfer to stop. The tape control disconnects from
the channel and proceeds independently to the completion of the operation.
When the operation is completed, the tape control tries to re-establish connection
with the channel to transfer ending status.
If
addressed while completing the
operation, the tape control returns a 'busy' signal.
If
a Halt
1/0
instruction is executed after 'status in' and before tape motion is
started during a Write or Read, the operation is canceled, and Channel End,
Device End, and Unit Check are generated.
28
3803-2/3420 Subsystem Description
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