The text read off it
[06.02.05]
It should be noted that no true unit assignments may be requested. This follows from the fact that any given unit may be performing a SYStem function, and therefore not available for assignment as an object-time unit. However, the programmer is allowed to request relative units on channels. In explaining the meaning of relative units, the following example is given.
In sorting problems, a programmer may use many tapes, alternately reading several and then writing several. From the logical flow the program, the programmer may decide that 3 files (for example) should logically be on one channel and 3 other files should logically be on another channel for best operating efficiency. The programmer might then use the symbolic unit designations A(1), A(2), and A(3) to refer to three of the files and B(1), B(2), and B(3), to refer to the other three files. The numbers (1), (2), and (3), are the programmers' indicies or relative unit assignment. The loader tape-assignment-routine will try to find three available units on channels A and B. Available, used in this sense, means units attached to the computer and not referenced in the SYStem units table (i.e. not used by the system). Using the distributed version of the SYStem units table as an example, the following assignment would be made:
| Symbolic | Assignment Made by Loader |
|---|---|
| A(1) | A5 |
| A(2) | A6 |
| A(3) | A7 |
| B(1) | B4 |
| B(2) | B5 |
| B(3) | B6 |
(This assumes there are at least 7 units attached on Channel A, and at least 6 units attached on Channel B).
The logical process by which symbolic unit requests are converted to physical unit assignments in the order of priority is:
- All card units, SYStem units, and true-channel relative units are assigned first. The symbolic units OU, OU1, OU2, IN, IN1, IN2, PP, PP1, and PP2 when assigned to SYStem units take on the characteristics of the SYStem units. They are considered multi-reel alternating units with no labels, and any programmer rewind options are ignored. In this respect, for example, OU, OU1, OU2, are synonomous. The symbolic units UT1, UT2, UT3 should use the 'DEFER' mounting option as SYSUT1, SYSUT2, SYSUT3 are used by the loader. (Mounting instructions occur at object-time OPEN).