1991, and support ended in 2004. In the B4900 and later machines, integer operations of 10 digits or less were now handled in parallel; only longer operands continued to use the serial method. And all floating point operations were limited to 17 digits of precision. Later Medium Systems machines added an accumulator register and accumulator/memory instructions using 32-bit, 7-digit integers and 48-bit or 80-bit floating point values, all aligned on 16-bit word boundaries. 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 internally, and was coded in assembly language, not an ALGOL derivative. 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 five digits to six digits, and four 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 achieved 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 assembly language 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 system calls using the BCT instruction and exposed to the languages (COBOL FILL FROM/INTO). This was unheard