Dear Erik,
Thank you for your input. I think I got it now. In the other word, Keil startup.a51 uses "dynamic" Stack by putting it at the end of "used" memory as you said. For instances, if I have 5 variables declared in my code, then Stack will start right after the 5th variable, am I right? And 1 byte reserved is a pointer not Stack itself.
I also reallized one thing that when it clear memory, it didn't clear the last byte of memory, perhaps this is for pointer.

IF IDATALEN <> 0
                MOV     R0,#IDATALEN - 1
                CLR     A
IDATALOOP:      MOV     @R0,A
                DJNZ    R0,IDATALOOP
ENDIF

Once again, thanks a lot for all inputs. Have a great day.

Best regards,
T.L