Technical data

Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
582 Manual, 05/2010, A5E02789666-01
27.2.3.5 Block Stack
If processing of a block is interrupted by the call of another block or by a higher priority class
(interrupt/error servicing), the B stack stores the following data:
• Number, type (OB, FB, FC, SFB, SFC), and return address of the block that was interrupted.
• Numbers of the data blocks (from the DB and DI register) that were open when the block was
interrupted.
Using this data, the user program can then be resumed after the interrupt.
If the CPU is in STOP mode, you can display the B stack with STEP 7 on a programming device.
The B stack lists all the blocks that had not been completely executed when the CPU changed to
STOP mode. The blocks are listed in the order in which processing was started (see figure below).
Data Block Registers
There are two data block registers. These contain the numbers of opened data blocks, as follows:
• The DB register contains the number of the open shared data block
• The DI register contains the number of the open instance data block.