Basics of program execution
6.2 Asynchronous instructions
CPU 1516pro-2 PN (6ES7516-2PN00-0AB0)
52 Operating Instructions, 11/2019, A5E35873416-AC
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:
● Jobs for an asynchronous instruction are started while other jobs for that instruction are
still running.
● The maximum number of simultaneously running jobs for the instruction is not exceeded.
The figure below shows the parallel processing of two jobs of the WRREC instruction. The
two instructions are executed simultaneously for a certain duration.
Figure 6-2 Parallel processing of the asynchronous instruction WRREC
Assigning calls of an instruction to a job
To execute an instruction over multiple calls, the CPU must be able to uniquely relate a
subsequent call to a running job of the instruction.
To relate a call to a job, the CPU uses one of the following two mechanisms, depending on
the type of the instruction:
● Using the instance data block of the instruction (for "SFB" type)
● Using job-identifying input parameters of the instruction. These input parameters must
match in each call during processing of the asynchronous instruction.
Example: A job of the "Create_DB" instruction is identified by input parameters
LOW_LIMIT, UP_LIMIT, COUNT, ATTRIB and SRCBLK.
The following table shows which input parameters you use to identify which instruction.
Table 6- 2 Identifying input parameters for asynchronous instructions
LOW_LIMIT, UP_LIMIT, COUNT, ATTRIB, SRCBLK
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