Unisys History Newsletter

Volume 3, Number 5
October 1999

Burroughs Third-Generation Computers

by George Gray

The B5500

It was just one year after the first B5000 customer delivery that IBM announced the System/360, and four months later (August 1964) Burroughs responded with its announceme nt of the B5500. Even though it was not his product, Irven Travis, director of the Defense, Space, and Special Systems group (otherwise known as the Great Valley Laboratories) near Paoli in suburban Philadelphia had been very impressed with the potential of the B5000 and convinced Burroughs Corporation president Ray Macdonald to authorize work on an improved version. The circuitry of the B5500 was three times faster than that of the B5000, and this increase in speed, coupled with the use of disks in place of drums, made the B5500 a success for the company. To improve the performance of COBOL programs, the B5500 added hardware instructions for addition and subtraction of decimal fields. Burroughs also adjusted to the reality of the marketplace and provided a FORTRAN compiler for the B5500.

The B5000 at NASA in Huntsville, Alabama was upgraded to a B5500 in November 1964. Most of the other B5000 customers upgraded to B5500s and those with B5000s on order switched to B5500s. The U.S. Air Force Academy installed a dual-processor B5500 in early 1965. Orders came in faster than had been forecast, and by the time the last one was manufactured in 1970, about 220 had been shipped. The excellence of the B5500's programming environment enabled Burroughs to get a number of IBM users to switch to the B5500. One of these was Carrier Corporation, a manufacturer of air conditioning equipment, which converted a set of 86 COBOL programs from the 360 to the B5500 in ten weeks and found it easy to do. Other B5500 customers included the University of Washington (which wrote a compiler for the APL programming language), the University of Denver, Stanford University, Monash University in Australia, Drexel Institute of Technology, the City of Montreal, the British General Post Office, and the U.S. Bureau of Mines. B5500s lingered on through the late 1970s at various universities where the multiprogramming and virtual memory capabilities gave them a distinct advantage over low-end IBM 360s and various smaller computers. In 1978, the University of California Santa Cruz had two B5500s, one donated by Burroughs and one by the First National Bank of San Jose, which it used for research and student computing. The University was quite happy with the B5500 and planned to use it as long as it could maintain the hardware.

The B2500 and B3500

While the B5500 was an excellent large scale system, something was needed to replace the B200 and B300 small and medium systems. Burroughs announced the B2500 and B3500 in March 1966. They had been developed in Pasadena and used many features of the B5500, such as the hardware stack and multiprogramming capabilities, and added other features to enhance the execution of COBOL programs. According to Dave Dahm, they were, in fact "designed with the explicit idea of being the target of a COBOL compiler." Burroughs offered two operating systems: the Basic Control Program (BCP) for one-at-a-time batch processing and a more sophisticated Master Control Program, patterned after that of the B5500, which supported multiprogramming. Under MCP, a B3500 could process up to 16 programs simultaneously.

Burroughs saw the IBM 360/30 and 360/40 as the principal competition for the B2500 and B3500, so the company decided to make their i/o systems IBM-compatible by using IBM's EBCDIC data code and many IBM file structures. There was no attempt at machine language compatibility (the strategy of RCA), but Burroughs did provide simulators for IBM's older 1401 computers. For its own customers there was a translator which would convert B300 assembler programs to the B2500 or B3500. Burroughs was successful in selling B3500s to military customers. In December 1967, the company won a $60 million contract to supply 150 of them for the U.S. Air Force base logistics project. The contract had originally been awarded to IBM, but the Department of Defense re-evaluated the bids after a protest by Honeywell. Burroughs also sold 43 to the U.S. Navy Systems Command (in 1971) and four modified verions to the U.S. Army for a mobile communications system. In all, 990 B3500s were delivered to civilian and military customers.

The B6500

Further improvements to the B5500 resulted in the B6500, announced in 1966 but not delivered until 1969, almost a year behind the original schedule, as a result of problems with electronic signal interference in its circuitry. Even then, hardware troubles continued for another year before the system settled down. The B6500 continued the development of some of the unique features of the B5000/B5500 architecture. One of these features was the memory tag, which categorized the contents of each word in memory. The designers of the B6500 expanded it to three bits (from one on the B5000), so that it could indicate whether a word contained an instruction, control information, or data. This was used as an additional memory protection feature, preventing an erring program from trying to execute data as an instruction or treating an instruction as data. Furthermore, for a data word the tag identified the data format: fixed-point, floating-point, or character/decimal. This made possible a very compact instruction set, because a single add instruction could determine from the tag what type of data was involved. (The B5000 had two add instructions, since a one-bit flag did not distinguish between single and double floating.) Most other computers had multiple add instructions: one for fixed-point, one for single floating, one for double floating, and so on. Since fewer instructions were needed, the instruction's operation code was reduced from twelve bits to eight. The stack architecture introduced in the B5000 meant that simple word-oriented instructions did not need to have an operand field, since the operands were always at the top of the stack. More complex instructions ranged up to 72 bits in length. Each 8-bit part of an instruction was referred to as a syllable, since six could be put into one word.

The B6500 MCP also introduced a new input/output handler. On the B5500 each language compiler had its own file formats, with the result that files created by ALGOL programs could only be read by ALGOL programs, those created by COBOL could only be read by COBOL, and so on. The new input/output handler allowed any program to read any file. Unfortunately, the original implementation was very slow, in many cases taking twice the amount of time as on the B5500. The i/o handler was rewritten and greatly improved for the new model in the series (B6700). The B6500 was the first model in the series where the MCP operating system provided full time-sharing capabilities. Some time-sharing had been retrofitted to the B5500 MCP, but the B6500 version was much more powerful.

The B8500 and Its Cousins

In the meantime, the Defense, Space and Special Systems group announced the B8500, a machine which had its origins in a December 1961 proposal to the Atomic Energy Commission Livermore Laboratory for a high-speed scientific computer to replace the LARC. The design was reworked through the early 1960s and proposed again to General Electric's Knolls Atomic Power Laboratory in April 1964. The announced version of 1966 was an attempt to blend the features of the B5500 and the D825, utilizing monolithic integrated circuits in the processors, core for main memory, and thin-film memory for high speed work areas. Like the D825, the design provided for separate instruction processors, i/o processors, and memory modules. There could be any combination of instruction and i/o processors up to a maximum of 16. The memory would be in modules of 16,384 48-bit words, again with a maximum of 16 (262,144 words), having a design cycle time of 500 nanoseconds. The B8500 was to use the same disk and tape equipment as the B2500, B3500, and B5500. Burroughs wrote a new operating system (Executive Scheduling Program, ESP) based on the AMOS and AOSP operating systems of the D825. Following the precedent of the B5000 and B5500, all system software was written in ALGOL. A new building was added to the group east of Paoli to be the factory for the B8500; since it was across the township line in Tredyffrin Township, it became known as the Tredyffrin facility, or Tredy for short.

As the details of the B8500's design were announced, it was widely regarded as a very sophisticated machine. It was also very expensive: when the University of Wisconsin ordered a B8500 with three instruction processors in the spring of 1966, the contract price was $14 million. The installation was scheduled for 1968, and a B5500 was delivered in 1967 as an interim machine. U.S. Steel Corporation and the National Provincial Bank in England also ordered B8500s. Unfortunately, Burroughs encountered serious problems in getting the processor and memory components to work reliably, and in the summer of 1968 the company said that shipments would be delayed. The problems could not be overcome in any cost-effective manner, so at the end of 1968, the company announced that it would ship two B6500s to U.S. Steel and one B6500 to the National Provincial Bank instead of their B8500s. As late as the spring of 1970, Burroughs still hoped to deliver a B8500 to the University of Wisconsin, but later that year it gave up and canceled the order. The University of Wisconsin bought a UNIVAC 1108. The prototype B8500 machines never left the Tredyffrin factory. Looking back on the project a few years later, a Burroughs engineer said that the B8500 never could run as fast as the transistor IBM 7094.

In 1964 Burroughs had also completed the D830 which was another variation of the D825 designed specifically for real-time applications, such as airline reservations. Burroughs designated it the B8300 after TWA ordered one in September 1965. A system with three instruction processors was installed at TWA's reservations center in Rockleigh, New Jersey in 1968. The system, which was called George, was intended to support some 4000 terminals, but even after a fourth processor was added it couldn't handle the workload. TWA canceled the project in 1970, acquiring one IBM 360/75 and two 360/65s and the PARS software for its reservations system. TWA sued Burroughs for non-fulfillment of the contract, but Burroughs countersued, stating that the basic system did work and that the problems were in TWA's applications software. The two companies reached an out-of-court settlement.

Not all of the descendants of the D825 were as troubled as the B8300 and B8500. Burroughs developed a half-size version of the D825 called the D82, which cut the word size from 48 to 24 bits and had a simplified instruction set. The D82 could have up to 32,768 words of core memory and continued the use of separate instruction and i/o processors. Burroughs sold a D82 to Air Canada to handle reservations for trips originating in Montreal and Quebec. This design was further refined and made much more compact as the D84 which was completed in 1965. A D84 processor/memory unit with 4096 words of memory occupied just 1.4 cubic feet. This system was used successfully in two military projects: field test systems used to check the electronics of the Air Force F-111 fighter plane and systems used to control the countdown and launch of the Army's Pershing missile.

The 700 Series

When Burroughs developed successors to the B5500 and B6500, it skipped the 600 numbers (perhaps to avoid confusion with Control Data's CDC 6600 computer) and announced the 700 series in October 1970. Quick deliveries of the 5700 and 6700 did much to offset the bad publicity from the delays on the 6500 and the 8500/8300 failures. The first B6700s were installed during 1971 and most 6500s in the field were upgraded to become 6700s. A further rewrite of the MCP i/o handler did away with the slowness which had been introduced with the B6500 version. The MCP for the B6700 also introduced dynamic linking, whereby procedures could be linked to a program at runtime, instead of having to be statically linked prior to the execution of a program. The B6700 line started out with one CPU and one i/o processor and could be expanded up to a maximum of three CPUs and two i/o processors. The 6500 and 6700 had been designed and produced in California, but now the east coast group also became involved.

The designers in Tredyffrin completed the prototype B7700 which had more powerful processors and cache memory at the very end of 1971. Originally the B7700 was limited to a maximum of four CPUs and two i/o processors, but later models (introduced in 1976) could have any combination of CPUs and i/o processors up to a maximum of eight. All B6700 and B7700 models could go up to 6 million bytes of memory; some used a combination of semiconductor and ferrite core memory, while others were still all ferrite core. The B6700 and B7700 also provided a new hardware mode to support vector operations. This improved the capability of the processor to do iterative operations on data vectors and arrays. The 6700 and 7700 were both using the MCP operating system, but there were minor differences between the two versions and there was a tendency for them to diverge further. In 1983 the two were finally merged into common source code, although certain hardware input/output status codes continued to differ, and customers had the option to tailor the MCP by setting various parameters at the time they compiled it.

More than 20 B6700s were delivered during the first nine months of 1971, and at the end of the year the New Zealand university system placed an order for five B6700s. Other B6700s were acquired by the Florida Department of Health and Rehabilitative Services, the Defense Logistics Services Center (Battle Creek, Michigan), the City of Jacksonville, the U.S. Geological Survey (Sioux Falls, South Dakota), the Air Force Military Personnel Center, the U.S. Bureau of Mines, and the New York Clearing House Association, which handled inter-bank transfers. The first customer shipment of a B7700 was to one of the U.S. telephone companies in 1973. In later years, 7700s were installed at the Federal Home Loan Bank of Chicago, the International Monetary Fund, Midland Bank, Pacific Bell, Michigan Bell (which had five dual-processor machines), the Brazilian social security department, the University of Eindhoven (Netherlands), and the Banque Bruxelles in Belgium.

Burroughs also brought out new machines for its medium series: the B2700, B3700, and B4700 to take the place of the B 2500 and B3500. The new models could run existing B2500/B3500 programs without change. The B4700, which was twice as fast as the B3500, had from one to four CPUs, and the virtual memory capability was standard. Prices for the B4700 ranged from $650,000 for a one-CPU model up to $4,500,000 for four CPUs. The old batch BCP operating system was dropped, and the multiprogramming operating system was enhanced to handle up to 80 programs and renamed MCP V. The new File Protect Memory feature allowed multiple programs to access the same data file simultaneously in a fashion parallel to the database management systems of larger computers. Starting with the B4700, Burroughs decided to follow the example of the large systems and not provide an assembler in the medium series.

In the small computer market, the company introduced the B1700 in 1972 to compete with IBM's System/3. For a small machine the B1700 was very remarkable in that it provided virtual memory capabilities and variable micrologic in the processor, so that it could behave as either a word or character-oriented machine. Burroughs provided interpreters for COBOL, FORTRAN, BASIC, and RPG as well as emulators for the older B200 and B300 machines. The various models of the B1700 had from 24,000 to 378,000 bytes of memory and sold for $22,000 to $87,000. Sales of the 1700 series were relatively strong. The Yugoslavia n State Bank ordered 74 of them for use at its branches and in all, over 1300 B1700s were sold during its first three years.

IBM's 1972 announcement of virtual storage for its large 370 series machines was taken by Burroughs as validation of the virtual memory concept. The company held a well-publicized tenth birthday party for the B5000 at the old ElectroData plant in Pasadena, where a B5500 (which had been rebuilt from a B5000) was still in operation. By the early 1970s, both the large and medium Burroughs computers had inspired a high degree of loyalty among their users, who were proud of the advanced features incorporated in the Burroughs architecture. It would still be several more years before IBM really moved into multiprocessing, while it was commonplace to Burroughs users.