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Two types of execute cycles are used by the CPU. A-cycles
are used to address main storage for the source fields and
B-cycles are used to address main storage for the result
field. If only one field is involved with the instruction,
B-cycles are used to address main storage.
Sequential Instruction Execution
• Instructions are located in consecutive, ascending main
storage locations.
• Instruction Address Register {IAR) is incremented by
1 each instruction cycle.
The CPU performs computations in a step-by-step procedure
similar to manual accounting. However, while many steps
may be combined in a manual operation, the CPU requires
a separate operation for each step. For instance, when
figuring net pay in a manual payroll operation, all the de-
ductions could be added together in one step. The CPU
must add each deduction into the deduction total in a
separate operation (Figure 1-6).
The example shown would probably require three steps in
the CPU. Since it would probably be desirable to retain the
federal tax figure, the total deductions would be calculated
in a separate location. Therefore, the first step would be to
reset the total deduction field to zeros and add the federal
tax into the total deduction field (zero and add zoned).
Then in separate steps the state tax and United Fund fields
would be added to the total (add zoned decimal).
Manual
CPU
Federal Tax
17.50
Federal Tax
17.50
State Tax
6.30
State Tax
6.30
United Fund
1.50
Total
23.80
Total Deductions
25.30
United Fund
1.50
Total Deductions
25.30
Figure 1-6. Step-By-Step Processing
5410 TO
1-9
IJ
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