Burroughs large systems

The Burroughs Large Systems Group produced a family of large 48-bit mainframes using stack machine instruction sets with dense syllables.E.g., 12-bit syllables for B5000, 8-bit syllables for B6500 The first machine in the family was the B5000 in 1961. It was optimized for compiling ALGOL 60 programs extremely well, using single-pass compilers. It evolved into the B5500. Subsequent major redesigns include the B6500/B6700 line and its successors, as well as the separate B8500 line.

In the 1970s, the Burroughs Corporation was organized into three divisions with very different product line architectures for high-end, mid-range, and entry-level business computer systems. Each division's product line grew from a different concept for how to optimize a computer's instruction set for particular programming languages. "Burroughs Large Systems" referred to all of these large-system product lines together, in contrast to the COBOL-optimized Medium Systems (B2000, B3000, and B4000) or the flexible-architecture Small Systems (B1000).

Background

Founded in the 1880s, Burroughs was the oldest continuously operating company in computing ( Elliott Brothers was founded before Burroughs, but did not make computing devices in the 19th century). By the late 1950s its computing equipment was still limited to electromechanical accounting machines such as the Sensimatic. It had nothing to compete with its traditional rivals IBM and NCR, who had started to produce larger-scale computers, or with recently founded Univac. In 1956, they purchased ElectroData Corporation and rebranded its design as the B205.

Burroughs' first internally developed machine, the B5000, was designed in 1961 and Burroughs sought to address its late entry in the market with the strategy of a completely different design based on the most advanced computing ideas available at the time. While the B5000 architecture is dead, it inspired the B6500 (and subsequent B6700 and B7700). Computers using that architecture were still in production as the Unisys ClearPath Libra servers which run an evolved but compatible version of the MCP operating system first introduced with the B6700. The third and largest line, the B8500, had no commercial success. In addition to a proprietary CMOS processor design, Unisys also uses Intel Xeon processors and runs MCP, Microsoft Windows and Linux operating systems on their Libra servers; the use of custom chips was gradually eliminated, and by 2018 the Libra servers had been strictly commodity Intel for some years.

B5000

The first member of the first series, the B5000, was designed beginning in 1961 by a team under the leadership of Robert (Bob) Barton. It had 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 operationsThere were security issues
* 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 the high three bits of T and on the Mark Stack FlipFlop (MSFF), as shown in B5x00 Relative Addressing.

Language support

The B5000 was designed to exclusively support high-level languages. This was at a time when such languages were just coming to prominence with FORTRAN and then COBOL. FORTRAN and COBOL were considered weaker languages by some, when it comes to modern software techniques, so a newer, mostly untried language was adopted, ALGOL-60. The ALGOL dialect chosen for the B5000 was Elliott ALGOL, first designed and implemented by C. A. R. Hoare on an Elliott 503. This was a practical extension of ALGOL with I/O instructions (which ALGOL had ignored) and powerful string processing instructions.
Hoare's famous