Erik:
"Keil automatically set the stack pointer at end of used RAM."

Scott:
"Than Why have the stack text field under targets, Option, BL51"

What Erik meant was, of course, "by default, Keil automatically set the stack pointer at end of used RAM."

The STACK Linker Directive lets you override this default
Read the description in the manual for cases where you might want to do this, and warnings if you do!