Siemens CPU 948, Programming Manual

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8.3.2
Block Address List in
Data Block DB 0
Data block DB 0 is located in the system RAM of the CPU (beginning
at address E E200H). It contains a list with the start addresses of all
blocks in the user memory of the CPU. The system program generates
(COLD RESTART) or checks (WARM RESTART) this list after
power up; it updates it automatically when you use a programmer to
enter or change blocks.
Address list start addresses
DB 0 has a separate, reserved address list of 256 words in each type of
block. Blocks that are not loaded or have been deleted have the start
address ’0’.
The start addresses of each block address list are specified (see Section
8.2.1).
Block start addresses
The block start addresses in the address lists always point to the first
word after the block header:
••
with data blocks, to data word DW 0
••
with logic blocks, to the first STEP 5 statement (in FBs to the ’JU’
operation before the name and the parameter list).
Since each block is located at a paragraph address (16 word
boundary), each address list entry in DB 0 is restricted to one word
with bits number 4 to 19 of the address.
Location of block
addresses in DB 0
n = E E400H (start address of the PB address list)
Address PB 0
Address PB 1
Address PB 2
Address PB 178
Address PB 179
If ’0’ is entered as
the address, the
block is not
loaded.
DB0
n
n+1
n+2
n+178
n+179
15 0
Fig. 8-6 Block addresses in DB 0
User Memory Organization in the CPU 948
CPU 948 Programming Guide
C79000-G8576-C848-04
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