Memory Parameters - GE PACSystems RX7i Reference Manual

Chapter 3. CPU Configuration

3.2.6 Memory Parameters

The PACSystems user memory contains the application program, hardware configuration (HWC),
registers (%R), bulk memory (%W), analog inputs (%AI), analog outputs (%AQ), and managed memory.
Managed memory consists of allocations for symbolic variables and I/O variables. The symbolic variables
feature allows you to create variables without having to manually locate them in memory. An I/O variable
is a symbolic variable that is mapped to the inputs and outputs of a module in the hardware
configuration. For details on using symbolic variables and I/O variables, refer to PACSystems RX7i, RX3i
and RSTi-EP CPU Programmer's Reference Manual, GFK-2950 Chapter 4.
The amount of memory allocated to the application program and hardware configuration is
automatically determined by the actual program (including logic C data, and %L and %P), hardware
configuration (including EGD and AUP), and symbolic variables created in the programming software. The
rest of the user memory can be configured to suit the application. For example, an application may have
a relatively large program that uses only a small amount of register and analog memory. Similarly, there
might be a small logic program but a larger amount of memory needed for registers and analog inputs
and outputs. Note that the content of reference memory is cleared any time the size of reference
memory is changed.
Appendix B provides a summary of items that count against user memory.
Calculation of Memory Required for Managed Memory
The total number of bytes required for symbolic and I/O variables is calculated as follows:
[((number of symbolic discrete bits) × 3) / (8 bits/byte)]
+ [((number of I/O discrete bits) × Md) / (8 bits/byte)]
+ [(number of symbolic words × (2 bytes/word)]
+ [(number of I/O words) × (Mw bytes/word)]
Md = 3 or 4. The number of bits is multiplied by 3 to keep track of the force, transition, and value of each
bit. If point faults are enabled, the number of I/O discrete bits is multiplied by 4.
Mw = 2 or 3. There are two 8-bit bytes per 16-bit word. If point faults are enabled, the number of bytes is
multiplied by 3 because each I/O word requires an extra byte.
Calculation of Total User Memory Configured
The total amount of configurable user memory (in bytes) configured in the CPU is calculated as follows:
Total managed memory (bytes)
+ total reference words × (2 bytes/word)
+ [if Point Faults are enabled] (total words of %AI memory + total words of %AQ memory) × (1 byte / word)
+ [if Point Faults are enabled] (total bits of %I memory + total bits of %Q memory) / 8 bits/byte)
Note: The total number of reference points is considered system memory and is not counted
against user memory.
120 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD