Features and internals
One feature of PRIMOS was that it, like UNIX, was largely written in a high level programming language (with callable assembly language library functions available). At first, this language was FORTRAN IV, which was an odd choice from a pure computer science standpoint: no pointers, no if-then-else, no native string type, etc. FORTRAN was, however, the language most known to engineers, and engineers were a big market for Prime in their early years.
Very early versions of PRIMOS (revision 6) were originally called DOS (PRIMOS 2) and later DOSVM (PRIMOS 3), but starting with PRIMOS 4, on the P400 system, PRIMOS was the name that stuck. There were many major releases of PRIMOS. The last official revision (24.0.0.R52) was released July 3, 1997. By this time, a company called Peritus (which employed a number of ex-Prime engineers) was maintaining PRIMOS.
From Revision 19, major portions of PRIMOS were written in the languages SPL and Modula-2, the usage of the Prime Macro Assembler (PMA), FORTRAN IV and PL/P declined considerably around this time. Programs were guaranteed to run on all current Prime processors (subject to sufficient resources being available), as well as all subsequent Prime processors.
One thing rarely mentioned is that the language compilers used large amounts of resources, so most development and instructional sites implemented compiler queues, while undergrads figured out how to manipulate the queue.
Lore found in the printer bin: "In heaven they have instant access computers, in hell they have Primes."