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Program Execution Levels
Nesting other levels
4 - 6
When an event occurs, which requires higher priority processing, the
current level is interrupted by the system program and the higher
priority level is activated.
This occurs in the following situations:
• • at error levels:
• • all other levels:
Note
A maximum of 5 error organization blocks can be nested. If 5
error levels are activated simultaneously, this causes an ISTACK
overflow and the CPU changes to the HARD STOP.
A specific program execution level is assigned to one or a group of
organization blocks which are called by the system program after an
event. If, for example, OB 9 is called to process a time-controlled
interrupt, the program execution level TIMED INTERRUPTS is
activated.
After the system program calls an organization block, the CPU
executes the STEP 5 statements is contains. The current register
record is saved in the ISTACK and a new register record is set up
(register: ACCU 1 to 4, block stack pointer, block address register,
data block start address, data block length, step address counter, base
address register and the interrupt condition code words ICMK and
ICRW).
If "normal" program execution is interrupted by the occurrence of an
event, following the execution of the OB, the CPU continues the
program execution at the point of interruption (including all the blocks
nested there) as long as no stop is programmed in the OB.
always at operation boundaries,
at block or operation boundaries
(depending on the setting in DX 0
refer to Chapter 7)
CPU 948 Programming Guide
C79000-G8576-C848-04
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