The CALL_TABLE subroutine should be easy to see what it does. I suggest you load it into a simulator program and single step through it and watch all the registers and stack contents to see how it works.

From the comments supplied in the code itself and having a instruction code manual for the 8051 it should be possible to learn it.

I could try to write paragraphs about it....but if you trace it and figure it out on your own you will learn it so as to remember it for the rest of your life.

Michael Karas