A current operating system such as Solaris.RTM. X86 is adapted to run a user program such as a Common Object File Format (COFF) executable program which was designed to run on a retrograde operating system such as an older version of UNIX.RTM.. The operating system is adapted to include an emulator module or COFF interpreter which emulates retrograde system calls that are unique to the COFF system. A Local Descriptor Table (LDT) which is constructed for the COFF program includes a main system call gate and an alternate system call gate that initially both point to a system call entry point in the operating system kernel. Prior to running a COFF program, the main system call gate is revectored to point to the COFF interpreter, such that all system calls generated by the COFF program are directed to the COFF interpreter. The COFF interpreter redirects current system calls which are compatible with the current operating system to the kernel via the alternate call gate. Preferably, current system calls in a dynamic library are changed to point to the alternate call gate to minimize changes to the COFF interpreter.