Siemens Sinumerik 840D sl Programming Manual page 211

Adjustable reload memory (FIFO buffer)
A reload memory is required in the NCK in order to run a program in "Execution from external
source" mode (main program or subprogram). The size of the reload memory is preset to 30
Kbytes and, like all other memory-related machine data, can only be changed to match
requirements by the machine manufacturer.
One reload memory must be set for each program (main program or subprogram) to run
concurrently in "Execution from external source" mode.
RESET, POWER ON
RESET and POWER ON cause external subprogram calls to be interrupted and the
associated load memory to be erased.
A subprogram selected for "Execution from external source" remains selected for "Execution
from external source" after a RESET/part-program end. A POWER ON deletes the selection.
References
For further information about "Execution from external source", see:
Function Manual, Basic Functions, Mode Group, Channel, Program Operation, Reset
Behavior (K1)
Job planning
Programming Manual, 02/2011, 6FC5398-2BP40-1BA0
Flexible NC programming
1.25 Subprogram technique
211