Basics of program execution
8.2 Asynchronous instructions
Distributed I/O system
System Manual, 09/2019, A5E03576849-AJ
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Parallel processing of asynchronous instruction jobs
A CPU can process several asynchronous instruction jobs in parallel. The CPU processes
the jobs in parallel under the following conditions:
● Several asynchronous instruction jobs are called at the same time.
● The maximum number of concurrently running jobs for the instruction is not exceeded.
The figure below shows the parallel processing of two jobs of the instruction WRREC. The
two instructions are processed in parallel for a specific period here.
Figure 8-2 Parallel processing of the asynchronous instruction WRREC
Assignment of call to job of the instruction
To execute an instruction over multiple calls, the CPU must be able to uniquely assign a
subsequent call to a running job of the instruction.
To assign a call to a job, the CPU uses one of the following two mechanisms, depending on
the type of the instruction:
● Via the instance data block of the instruction (in the case of the "SFB" type)
● The input parameters of the instruction identifying the job. These input parameters must
match in each call during processing of the asynchronous instruction.
Example: A "Create_DB" instruction job is identified by the input parameters LOW_LIMIT,
UP_LIMIT, COUNT, ATTRIB and SRCBLK.
The following table shows which instruction you can identify with which input parameters.
CREATE_DB LOW_LIMIT, UP_LIMIT, COUNT, ATTRIB,
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