PRIMOS is a discontinued operating system developed during the 1970s by Prime Computer for its minicomputer systems. It rapidly gained popularity and by the mid-1980s was a serious contender as a mainline minicomputer operating system. With the advent of PCs and the decline of the minicomputer industry, Prime was forced out of the market in the early 1990s, and by the end of 2010 the trademarks for both PRIME and PRIMOS no longer existed. Prime had also offered a customizable real-time OS called RTOS. Internals One feature of PRIMOS was that it, like UNIX, was largely written in a high level 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. The unusual choice of FORTRAN for the OS programming language had to do with the people who founded Prime. They had worked for Honeywell on a NASA project. FORTRAN was the language they had used both at NASA and, for many of them, at MIT. Honeywell, at that time, was uninterested in minicomputers, so they left and founded Prime, "taking" the code with them. They developed hardware optimized to run FORTRAN, including machine instructions that directly implemented FORTRAN's distinctive 3-way branch operation. Since Prime's hardware did not perform byte addressing, there was no impetus to create a C compiler. Late models of the hardware were eventually modified to support I-mode, and programs compiled in C. Later, around version 18, a version of PL/1, called PL/P, became the high level language of choice within PRIMOS, and the PL/P and Modula-2 languages were used in the Kernel. Furthermore, some new PRIMOS utilities were written in SP/L, which was similar to PL/P. The source code to PRIMOS was available to customers and, thanks to FORTRAN and PL/P, customers could reasonably modify PRIMOS as needed. For example, around 1990, the University of Salford in the UK, modified the PRIMOS running on its five 9955 systems so that undergraduates could no longer use the MESSAGE command, that wrapped the PRIMOS SMSG$() call, to send messages to other undergraduates, because online "chatting" using that command was becoming rife, tying up terminals from the limited pool available. Messaging using that