Operating System
The operating system was called MCP, for Master Control Program. It shared many architectural features with the MCP of Burroughs' Large Systems stack machines, but was entirely different inside, and was coded in assembler, not some high level language. Programs had separate address spaces dynamically relocated by a base register, but otherwise there was no virtual memory; no paging and no segmentation. Larger programs were squeezed into the limited code address space by explicit overlays. The nonresident parts of MCP were also heavily overlaid. Initially, code and data shared a single 300,000 digit address space. Later machines had separate million-digit spaces for program code and process data. Instructions' address fields were extended from 5 digits to 6 digits, and 4 more real index registers were added.
Early machines used Burroughs's head-per-track disk systems rather than the now-standard movable head platter disks. In one attempt to speed up MCP, its overlays were carefully laid out so that the likely-next overlays would soon arrive at their read head just after the current overlay completed. This was similar to time-dependent layout optimizations on early delay-line and drum computers. But this turned out to be impractical to maintain after software changes, and better results were consistently gotten with a totally randomized layout of all MCP overlays.
Other than the operating system itself, all system software was coded in BPL (Burroughs Programming Language), a systems programming language derived from ALGOL and Large System's ESPOL systems language. The initial COBOL compiler supported the ANSI 68 specification and supported the ENTER SYMBOLIC syntax to allow inline assembler coding, but lacked support for RELATIVE and INDEXED file support; these were later added into the ANSI 74 version of the compiler, which was released in 1982. MCP allowed programs to communicate with each other via core-to-core transmissions (CRCR) or by using storage queues (STOQ), implemented as OS supervisor calls using the BCT instruction and exposed to the languages (COBOL FILL FROM/INTO). This was unheard of except on the very largest IBM S/360 systems of the time, and even then it was a major operational headache to manage the interactions of the multiple program streams.
Read more about this topic: Burroughs B2500
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