S7 1200

Hi,

See the SIMATIC S7 S7-1200 Programmable controller under chapter "3.3.9 Changing the operating mode of the CPU".

As you can read there MRES makes a Memory Reset. There is long story about that but the summary under chapter "5.1.1 Operating modes of the CPU"  Clears it up:

"A memory reset clears all work memory, clears retentive and non-retentive memory areas,
copies load memory to work memory, and sets outputs to the configured "Reaction to CPU
STOP". A memory reset does not clear the diagnostics buffer or the permanently saved
values of the IP address."