mcs 51
today there is no such thing, the variety features in derivatives is so great that if you do not specify your derivative, even the best answer will be a guess.

since you force me to assume: Assuming that you have a derivative with 128 bytes of RAM, no more no less:
CJNE R0,#07FH,START_IN ; TO AVOID STACK OVER FLOW
should be
CJNE R0,#07FH,START_IN ; TO AVOID BUFFER OVER FLOW

that would avoid buffer overflow, but when you come to the 'RET' the stack would be overwritten and you would 'return' to the 'address' of your last data.

The content of your questions make it crystal clear that you need to study "the bible" (address in links <---) specifically chapters 2 and 3 before attempting to solve your problem.

Erik