Simply Stack in RAM with VHDL

Hi friends!, well, I have VHDL code attach (so simply), and I'd like add

two operations more, CALL and RETURN, so I need Stack Pointer (SP)

[register], it must be point at the end of the memory and stack should

start at high addresses and as it grows move to lower addresses, the SP

in memory will be store programcounter where jump when return. I use

this simple memory file (also attach as program.m, must be program.mif [problems with uploader]).

How I can do it?, I dont know how save programcounter in memory pointed

by sp.

Thanks in advance