The original version of MPE was written in a language called SPL (System Programming Language); later the name of the OS was changed to MPE/iX to indicate Unix interoperability with the addition of POSIX compatibility.
The original version of MPE was written in a language. This language was called SPL (System Programming Language). Later the name of the OS was changed to MPE/iX. This was to indicate Unix interoperability with the addition of POSIX compatibility.