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In the preceding example, initially the CSR command is executed to test the ready status of
the tape controller.
When the controller becomes ready, the 3 command words are loaded from
their temporary storage locations and put into locations 3, 4, and 5. The SEL command initiates
operation of the controller selector unit and the commands are automatically sent to the tape
controller.
Next, the NIS7 interrogates the controller selector to see if the last controller
select is finished. When the select has been finished the program returns to write a new record.
Tape Unit Conditions
Tapes contain a silve;r spot to signal the physical end of the tape. When detected by a photo-
electric cell within the tape unit, an indicator on the tape controller is set. The condition of the
indicator should be tested by programmed instructions after reading or writing each record.
If
the indicator is not set, normal processing will continue.
If
it is set, an end-of-tape branch
will jump into specified subroutines - normally rewinding the current reel and switching to a
new reel.
The end of file sentinel is the magnetic representation of the binary code 001111
preceded by an erased section of the tape 3-3/4 inches long.
During magnetic tape operations several other exceptional conditions may occur which are
secondary to the main processing job.
Handling of these exceptional conditions may be conven-
iently assigned to "executive routines."
These conditions are handled as branch conditions.
Branch Conditions
The branch conditions concerned with the tape controller may be tested by examining the bits
in the B-register after a CSR instruction.
When the particular bit is on, the condition is true.,
as shown below:
B-register
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
Any Error
Modulo 3 or 4 Error
Input/output Error
Tape Parity Error
Any Handler Rewinding
End of Tape
End of File
Controller
Ready
Echo
[ID£1J&~~lf
c:J
~@------------
V-50
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