Siemens S7-300 appendix Address areas of system memory, Process image, Address areas Description

Memory concept 4.1 Memory areas and retentivity

4.1.4Address areas of system memory

System memory of the S7 CPUS is organized in address areas (refer to the table below). In a corresponding operation of your user program, you address data directly in the relevant address area.

Address areas of system memory

Table 4-4	Address areas of system memory
Address areas		Description
Process image of inputs		At every start of an OB1 cycle, the CPU reads the values at the
		input of the input modules and saves them the process image of
		inputs.
Process image of outputs		During its cycle, the program calculates the values for the outputs
		and writes these to the process image of outputs. At the end of
		the OB1 cycle, the CPU writes the calculated output values to the
		output modules.
Flag bits		This area provides memory for saving the intermediate results of
		a program calculation.
Timers		Timers are available in this area.
Counters		Counters are available in this area.
Local data		Temporary data in a code block (OB, FB, FC) is saved to this
		memory area while the block is being edited.
Data blocks		See Recipes and measurement value logs

Reference

The address areas of your CPU are listed in the Instruction list for CPUs 31xC and 31x.

I/O process image

When the user program addresses the input (I) and output (O) address areas, it does not query the signal states of digital signal modules. Instead, it rather accesses a memory area in CPU system memory. This particular memory area is the process image.

The process image is organized in two sections: The process image of inputs, and the process image of outputs.

Advantages of the process image

Process image access, compared to direct I/O access, offers the advantage that a consistent image of process signals is made available to the CPU during cyclic program execution. When the signal status at an input module changes during program execution, the signal status in the process image is maintained until the image is updated in the next cycle. Moreover, since the process image is stored in CPU system memory, access is significantly faster than direct access to the signal modules.

CPU 31xC and CPU 31x, Technical data	4-5
Manual, Edition 08/2004, A5E00105475-05