There is a difference in the way S7-200 handles retentive memory vs. S7-300. Retentive memory meaning whether it retains it's value on power loss or the STOP -> RUN transition.
In the S7-200 check the System Block, and retentive memory (I don't remember the exact menu), and see if there are defined ranges for M or V memory. in S7-200, you define what is retentive (or not retentive).
In the S7-300, all DBs are retentive (by default), and the number of bytes of retentive M memory is specified starting with address MB 0. This is in the hardware configuration of the CPU, under "cycle/clock memory".
That said, you may not have to worry about this at all, but it's a good idea to check on it.
Basically, if you need retentive values in the S7-300, use DB or M within the specified range. If you need non-retentive values, M memory above the specified range.