Hello aamirawan;

In the S7-300 CPUs, you will work with 2 distinct memory areas:

- work memory, based on RAM chips, where the program is executed;

- load memory, which in your case, is the MMC, where the programming computer will load the program.

When you compile and dowload a program to the CPU, it is written to the load memory, then when the CPU goes to Run mode, portions of the program are loaded to work memory as required by the program exrcution sequence.

So the PLC program will always be written to the MMC, where it will be retained, the MMC being a flash-EPROM card.

Read section 4 of the following document, explaining this in more detail.

https://support.industry.siemens.com/cs/ww/en/view/7302326

Hope this helps,

Daniel Chartier