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4.3
Memory areas, addressing and data types
The CPU provides the following memory areas to store the user program, data, and
configuration:
● Load memory is non-volatile storage for the user program, the data, and the
● Work memory RAM is volatile storage for some elements of the user project while
The CPU provides for storage of 2048 bytes of retentive data. You can specify work memory
data values, such as data in a DB or bit memory (M), that are to be saved permanently each
time power is removed.
Retentive memory is non-volatile storage for a limited quantity of work memory values. The
retentive memory area is used to store the values of selected user memory locations during
power loss. When a power down occurs, the CPU by design has enough hold-up time to
retain the values of a limited number of specified locations. These retentive values are then
restored upon power up.
Note
When you download a project or an element of the project, the retentive memory is reset.
An optional SIMATIC memory card provides an alternative memory for storing your user
program or a means for transferring your program.
• Program card: The memory card takes the place of
• Transfer card: The memory card is used to transfer a
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Manual, 05/2009, A5E02486774-01
configuration. When a project is downloaded to the CPU, it is first stored in the Load
memory area. This area is located either in a memory card (if present) or in the CPU. This
non-volatile memory area is maintained through a power loss.
executing the user program. To improve system performance, the CPU copies some
elements of the project from load memory into work memory. This volatile area is lost
when power is removed and is restored by the CPU when power is restored.
CPU memory; all of your CPU functions are
controlled by the card. The memory card is required
to remain in the CPU.
stored project from the card to a CPU; then, the card
must be removed. In this way, you can transfer your
project to multiple CPUs without using STEP 7 Basic.
How the S7-1200 works
4.3 Memory areas, addressing and data types
33
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