u will easily find this in the tutorials and thousands of web pages on 8051 basics.
still, some contribution fm my side:
mov SP,#X initializes the Stack. It starts the stack from mem. location X. the stack grows upward from X with the PUSH commands. there is no limit for the stack to grow upward and hence it is our responsibility to see that the stack doesn't grow beyond the physical memory space available in the micro.

have a nice time.......