Technical data

1.5 Accessing Operand Areas and Memory Areas
STEP 5 operations use two different mechanisms for accessing
operand areas and the entire memory:
Relative addressing The majority of STEP 5 operations address a memory location
relative to the beginning of the operand area. If these operations are
used exclusively, code and data areas of the user program are
protected against unintentional overwriting. At the same time, the user
program is dependent on the CPU as long as the CPU has an
appropriate operand area.
Absolute addressing Some STEP 5 operations work with absolute addresses. These
operations can be used to access the entire memory area. They can
only be used in function blocks and should only be used with great
care due to the danger of data corruption. These operations are
dependent on the CPU used. However, there is no difference between
the CPU 928 and CPU 928B regarding these operations.
Current data block Data blocks are loaded into the user memory or the DB-RAM by the
system program. Their location depends on the memory space
available in each case. The lengths of the individual data blocks can
vary and are set when programming the data blocks.
The current data block is the data block whose starting address and
length are entered in special registers. This entry is made via a special
STEP 5 operation for calling or "opening" a data block (like the page
of book). Unless operations with absolute addressing are used, the
user program can only access the current data block. The following
data types are possible: single bits, bytes, words and double words.
Accessing Operand Areas and Memory Areas
CPU 928B Programming Guide
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