What's an in circuit emulator? Is it a trainer board?

It emulates the target cpu under the control of your PC. You can say it is a PC controlled 8051, where you can control the execution of software (your code). Highly useful in real time debugging.

. Does CALL actually use 2 bytes of stack space? Then is there a difference between LCALL and ACALL in terms of stack space?

Yes, CALL uses 2 bytes on stack. Whether it is LCALL or ACALL, it is in the same 64K of code memory. So 64K can be represented by 2 bytes, no matter what the call type is.

Is there a way to initialize the stack pointer to an external memory location?

NO, the Stack has to be inside internal RAM.

IA