Hello Mulderm;
From the best source I know, the manual S7-200_e.pdf:
Accessing Data in the Memory Areas
Variable Memory Area: V
You can use V memory to store intermediate results of operations being performed by the control logic in your program. You can also use V memory to store other data pertaining to your process or task. You can access the V memory area in bits, bytes, words, or double words:
Bit: V[byte address].[bit address] V10.2
Byte, Word, or Double Word: V[size][starting byte address] VW100
Bit Memory Area: M
You can use the bit memory area (M memory) as control relays to store the intermediate status of an operation or other control information. You can access the bit memory area in bits, bytes, words, or double words:
Bit: M[byte address].[bit address] M26.7
Byte, Word, or Double Word: M[size][starting byte address] MD20
Local Memory Area: L
The S7-200 provides 64 bytes of local memory of which 60 can be used as scratchpad memory or for passing formal parameters to subroutines.
Tip
If you are programming in either LAD or FBD, STEP 7--Micro/WIN reserves the last four bytes of local memory for its own use.
Local memory is similar to V memory with one major exception. V memory has a global scope while L memory has a local scope. The term global scope means that the same memory location can be accessed from any program entity (main program, subroutines, or interrupt routines). The term local scope means that the memory allocation is associated with a particular program entity.
The S7-200 allocates 64 bytes of L memory for the main program, 64 bytes for each subroutine nesting level, and 64 bytes for interrupt routines.
The allocation of L memory for the main program cannot be accessed from subroutines or from interrupt routines. A subroutine cannot access the L memory allocation of the main program, an interrupt routine, or another subroutine. Likewise, an interrupt routine cannot access the L memory allocation of the main program or of a subroutine.
The allocation of L memory is made by the S7-200 on an as-needed basis.
Hope this helps,
Daniel Chartier