In the olden days, working with a minicomputer I had to do the following:
1) make a 'call tree' and insert a 0 in the name of all subroutines that did not call any other subroutines, a 1 in the names of subroutines that only called subroutines level 0 etc.
2) allocate specific variable slots for level0, level1 etc.

This is a maintenance nightmare, every time you change something you have to verify that the level is the still the same.

However that was the only way I could come up with for a 'manual overlaying'.

Keil has made that process automatic for C and available for asm with the OVERLAYABLE keyword, at least for DATA.

I do not know if you can use the OVERLAYABLE for xdata variables, look it up in the manual, try it, and let us know.

Erik