1.3 The Programs in a CPU
The program existing on every CPU is divided into the following:
•• the system program
and
•• the user program.
System program
The system program organizes all the functions and sequences of the
CPU which do not involve a specific control task (refer to Fig. 1-2).
Update process image
of the inputs
Output process image
of the outputs
System
program
Call
user
processing
(inter-
faces)
Execute start-up
Handle errors
Communication with
the PG
Manage memory
Fig. 1-1 Tasks of the system program
The Programs in a CPU
CPU 948 Programming Guide
C79000-G8576-C848-04
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