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9.4.4
Accessing the Dual-Port
RAM Memory
Application
Sequence of page access
Changing the page register
CPU 948 Programming Guide
C79000-G8576-C848-04
Operations with the Base Address Register (BR Register)
With the following operations, you can access pages organized as
bytes or words using an absolute memory address. The absolute
address is the sum of the BR register content and the constant
contained in the operation (-32768 to 32767).
Between the addresses F F400H to F FBFFH, the global memory area
has a window for accessing one of a maximum of 256 memory areas
called dual-port RAM pages. One dual-port RAM page can occupy a
maximum of 2K addresses and can be organized in either bytes or
words. Before access to the dual-port RAM area, one of the 256 pages
must be selected by entering its number in the select register (page
address register ). The procedure of writing to the select register and
then accessing the dual-port RAM area cannot be interrupted.
Before you can access the dual-port RAM area (load/transfer), you must
select one of the 256 dual-port RAM pages. Do this by putting the
number of the dual-port RAM page that you want to open into
ACCU-1-L. Use the ACR operation to enter this number into the
CPU-internal dual-port RAM register. All dual-port RAM operations
that follow ACR write the contents of the dual-port RAM register into
the select register of the corresponding modules on the S5 bus before
dual-port RAM access.
• • The page register is retained when another block is called.
• • If the page register is changed in a block, its value is retained
when the program returns to the calling block at the end of the
block.
• • The page register is retained after nesting in another program
execution level.
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