an unusual architecture. It has been listed by the computing scientist John Mashey as one of the architectures that he admires the most. "I always thought it was one of the most innovative examples of combined hardware/software design I've seen, and far ahead of its time." The B5000 was succeeded by the B5500 (which used disks rather than drum storage) and the B5700 (which allowed multiple CPUs to be clustered around shared disk). While there was no successor to the B5700, the B5000 line heavily influenced the design of the B6500, and Burroughs ported the Master Control Program (MCP) to that machine. Features Hardware was designed to support software requirements Hardware designed to exclusively support high-level programming languages Simplified instruction set No Assembly language or assembler; all system software written in an extended variety of ALGOL 60 named ESPOL. However, ESPOL had statements for each of the syllables in the architecture. Partially data-driven tagged and descriptor-based design Few programmer accessible registers Stack machine where all operations use the stack rather than explicit operands. This approach has by now fallen out of favor. All interrupts and procedure calls use the stack Support for other languages such as COBOL Powerful string manipulation All code automatically reentrant: programmers don't have to do anything more to have any code in any language spread across processors than to use just the two shown simple primitives. Support for an operating system (MCP, Master Control Program) Support for asymmetric (master/slave) multiprocessing An attempt at a secure architecture prohibiting unauthorized access of data or disruptions to operations Early error-detection supporting development and testing of software A commercial implementation virtual memory, preceded only by the Ferranti Atlas. First segmented memory model System design The B5000 was unusual at the time in that the architecture and instruction set were designed with the needs of software taken into consideration. This was a large departure from the computer system design of the time, where a processor and its instruction set would be designed and then handed over to the software people. The B5000, B5500 and B5700 in Word Mode has two different addressing modes, depending on whether it is executing a main program (SALF off) or a subroutine (SALF on). For a main program, the T field of an Operand Call or Descriptor Call syllable is relative to the Program Reference Table (PRT). For subroutines, the type of addressing is dependent on