Nesting other levels
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:
always at operation boundaries,
••
all other levels:
at block or operation boundaries
(depending on the setting in DX 0
refer to Chapter 7)
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.
Program Execution Levels
CPU 948 Programming Guide
4 - 6
C79000-G8576-C848-04
Summary of Contents for CPU 948
Page 10: ...Contents CPU 948 Programming Guide 1 2 C79000 G8576 C848 04 ...
Page 32: ...Contents CPU 948 Programming Guide 2 2 C79000 G8576 C848 04 ...
Page 72: ...Data Blocks CPU 948 Programming Guide 2 42 C79000 G8576 C848 04 ...
Page 74: ...Contents CPU 948 Programming Guide 3 2 C79000 G8576 C848 04 ...
Page 154: ...Contents CPU 948 Programming Guide 4 2 C79000 G8576 C848 04 ...
Page 200: ...Contents CPU 948 Programming Guide 5 2 C79000 G8576 C848 04 ...
Page 308: ...Contents CPU 948 Programming Guide 7 2 C79000 G8576 C848 04 ...
Page 324: ...Examples of Parameter Assignment CPU 948 Programming Guide 7 18 C79000 G8576 C848 04 ...
Page 326: ...Contents CPU 948 Programming Guide 8 2 C79000 G8576 C848 04 ...
Page 370: ...Addressable System Data Area CPU 948 Programming Guide 8 46 C79000 G8576 C848 04 ...
Page 372: ...Contents CPU 948 Programming Guide 9 2 C79000 G8576 C848 04 ...
Page 486: ...Contents CPU 948 Programming Guide 11 2 C79000 G8576 C848 04 ...
Page 522: ...PG Functions via the S5 Bus CPU 948 Programming Guide 11 38 C79000 G8576 C848 04 ...
Page 524: ...Contents CPU 948 Programming Guide 12 2 C79000 G8576 C848 04 ...
Page 538: ...Contents CPU 948 Programming Guide 13 2 C79000 G8576 C848 04 ...
Page 546: ...List of Key Words CPU 948 Programming Guide Index 6 C79000 G8576 C848 04 ...