The Burroughs B5900 and E-Mode

 A bridge to 21st Century Computing

By Jack Allweiss  Copyright 2010

  ~1960 to 1970 ~1971 to 1980 ~1981to 1983 1984-1990
Small Systems       A3
Medium Systems     B5900 A9
Large Systems B5000/B5500




B6900 A12
Very Large Systems   B7700/B7800 B7900 A15



Early Stack Architecture


Late Stack Architecture Machines E-Mode microarchitecture



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Evolution of Burroughs Stack Architechture - Mainframe Computers

From the 1960’s until the early 90’s most mainframe manufacturers offered a range of computers systems. Most fit into the following general classifications

Small systems – Branch office or small business computer

Medium systems – Medium size business computer

Large Systems – Large organizations or heavy DP, Banks, Universities

Very Large Systems – the largest organizations, Corporations and Government

The chart shown illustrates how the Burroughs stack architecture eventually came to cover the complete spectrum of mainframe systems offered by Burroughs. The dates are approximate and there is overlap in systems not shown.

In the 1960’s Burroughs small and medium systems offering were the B200/B2000/B3000 series of decimal machines. In the 1970’s the B2000/B3000 series was consolidated to the Medium market and the B1000 series of bit addressable machines was introduced for the small market.

In the 1960’s Burroughs did not have a very large business computer offering. The B5000/5500//5700 Stack architecture computers made up the large system offering. Each machine had a slightly different instruction set.

In the 1970’s the B6000 and later B7000 Large and Very Large stack machines were introduced. They were based on the B5000 series but had a significantly upgraded (i.e. more complex and complete) instruction set. Code for the B5000 series had to be recompiled the OS (MCP) or Compiler code rewritten to run on these machines.


In the 1980’s the first E-mode machine, the B5900 was introduced. It was about 98% compatible with the B6000/B7000 series of the 70’s. In general user programs would run unaltered, but there were some OS changes, and Compiler tweaks to take advantage of new string instructions. The B6900 and B7900 were not microcode architectures, but were tweaked to be about 99% compatible with E-mode (they were between the Late Stack and E-Mode architectures).

The B5900 did not replace the B2000/B3000 machines in 1981, but was offered along with them. The B3000/B4000 series were produced into the mid 1980’s and the A series finally ended that architecture and the Pasadena Plant was closed.

The A series was based on E-mode. The low end machines were not only code compatible, but were about 90% microcode compatible with the B5900. The A3 was a CMOS implementation of the B5900 and replaced the B1000 series machines in 1983. They were designed by the same engineers who used to work in Santa Barbara but who were moved to Mission Viejo and worked with my group. Because of some of the technology differences, and the fact that E-mode was now a standard and received full corporate backing, we were able to make several improvements to the microcode architecture.

The A9-A15 were also fully microcode, used different technologies to achieve higher speed, the high end A15 had a lot of special hardware and did not share the microcode of the B5900, but was fully E-mode compatible. These machines were all designed in Mission Viejo, CA. or Paoli PA. Mission Viejo also developed a single chip e-mode machine called the Micro-A, but it was never really marketed.

The A series was the end of the fully Burroughs designed hardware machines. In the 1990’s microprocessors became very powerful and very cheap. The strategy changed to implement E-mode as an emulator on microprocessors, with some CMOS coprocessors to handle operations that were too slow when emulated. To get more performance, more microprocessors were added. Unisys engineers were at the forefront of multi microprocessor design architecture in the 1990’s.

In the 21st Century microprocessors became powerful enough to emulate E-mode without any custom CMOS hardware, so my understanding is the latest Unisys Mainframes rely entirely on emulation and commercially available parts (actually Unisys Mission Viejo engineering had an E-mode emulator running on a standard laptop that I saw around 1996, but this was for software development purposes only, it was not powerful enough to replace a mainframe). Multi processors is still a key component of the emulation, but Intel basically incorporated much of the Unisys multiprocessor technology in there offerings. I am not completely familiar with the current Unisys designs so a better source for this information would be a Unisys mainframe engineer.

Variants of the B5900

The original B5930 was housed in a full size ("A" size) cabinet with a separate console that could house two displays (SPO’s) and the Maintenance Interface Processor (MIP). After production began Arnold Spielberg headed up a project to cost reduce the B5930, it was called the B5920. The product was housed in the lowboy cabinet being used by the B3950 at Pasadena. The separate console was eliminated and the MIP was integrated into the cabinet. The SPO terminal could be placed on top of the computer or on a desk. The B5920 was introduced in 1982.

Burroughs Brazil began producing the B5900 in 1981. They loved the machine as it was perfect for there market. The B5900 went out of production in the US in 1983, but continued to be produced in Brazil until 1988. One of the issues Brazil faced was technology export controls, which did not let US companies produced there highest performance machines in Brazil. Extending the life of the B5900 was a way around this problem.

The original design of the B5900 allowed for three expansion cards. In the US this feature was never used, but in Brazil they actually did use this feature. One of the limitations of the B5900 performance is that it has only three top of stack registers. The B6800/B6900 had sixteen, which corresponded to registers called lambda/delta pairs. Lack of these registers was a significant performance limiter for the B5900. To boost performance the Brazilians designed a card that implemented all sixteen registers. This gave the machine about a 50% performance boost.

The second add on the Brazilians did was a math coprocessor. The B5900 did not have hardware multiply or divide units, so these operations were not particularly fast. The Brazilians added this function and increased that performance by several hundred percent.

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