Talk:Burroughs Large Systems

In this edit, a comment

On the B5000 there are 4 types of syllables, only one of which has an opcode, and on the B6500 an instruction may have multiple syllables. instruction syllable is a redirect to opcode.

was changed to

On the B5000 there are 3 types of syllables, only one of which has an opcode, on the B5500 an instruction has 2 types of syllables which appear very similar although may be different and on the B6500 an instruction has 4 syllables, however the instruction is not clear. instruction syllable is a redirect to opcode.

The Burroughs B5500 Information Processing Systems Reference Manual] says, on pages 5-1, that "...syllables are packed four to a core memory word (12 bits for each program syllable)." and, on page 5-2, that "In word mode, syllables are grouped into four categories: Descriptor Call, Operand Call, Literal Call, and Operator syllables."

It also indicates that:

• In all syllables, bits 10 and 11 contain the syllable category; presumably neither version of the comment considers those bits to be part of an opcode.
• The Literal Call (LITC) syllable uses bits 0-9 to contain "the integer value", the Operator syllable uses those bits "for determining the type of the operator syllable", and the Operand Call (OPDC) and Descriptor Call (DESC) syllables use those bits "to contain the index for relative addressing and to indicate the base address of the area which will be referenced", so presumably the Operator syllable is the only one with an opcode, the opcode being "the type of the operator syllable".

That seems to match what the original comment was saying.

However, that only describes word mode syllables. B5500 instruction processing is modal - the machine can either be in "word mode" or "character mode", and, in character mode, instructions are also made from 12-bit syllables, in which the lower 6 bits contain what appears to be an opcode.

The Operational Characteristics of the Processors for the Burroughs B 5000 says, on page 5-1 through 5-2, that:

• syllables are 12 bits long
• the processor has two modes, "word mode" and "character mode";
• the lower 2 bits indicate the syllable type for word mode syllables;
• the types are Operator, Literal, Operand Call, and Descriptor Call;
• the upper 10 bits of a Literal syllable are an immediate operand;
• the upper 10 bits of an Operand Call or Descriptor Call syllable are used as an index relative to contents of the R register;
• the upper 10 bits of an Operator are used to, among other things, indicate the operation;

and says, on page 5-9, that character mode syllables have an "operator code" in the lower 6 bits, which sounds very similar to the description of the B5500 syllables; I suspect that the two encode syllables the same way, unless there are, for example, some extensions in the B5500.

So I'm not seeing any indication that the B5000 and B5500 differ significantly here; I'm guessing that the B5500 is binary-compatible with the B5000.

As for the B6500, the Burroughs 6500 Information Processing Systems Reference Manual says, on page 6-1, that

A machine language program is a string of syllables that are normally executed sequentially. Each word in memory contains six 8-bit syllables.

On pages 6-2 through 6-3, it says that

Operations are grouped into 3 classes: Name call, Value Call, and operators. The two high-order bits (bits 7 and 6) determine whether a syllable begins a Value Call, Name Call, or operator (figure 6-3).

and figure 6-3 indicates that:

• Value Call operations contain 2 syllables, with the first syllable's upper 2 bits being 00;
• Name Call operations contain 2 syllables, with the first syllable's upper 2 bits being 01;
• operators contain 1 through 12 syllables, with the first syllable's upper bit being 1 and the value of the next bit, apparently, being 0 or 1.

On page 6-4, it says that the operator can either be word operators or string operators, so it sounds as if there's no notion of word mode or character mode, with the operator itself indicating whether it's a word or string operator.

This sounds not at all like what the comment says after the edit in question.

I'm reverting the comment change (and will see whether anything in this article or other Large System articles say anything about instruction formats). Future discussion should take place here. Guy Harris (talk) 07:12, 2 August 2022 (UTC)

B5000, B5500 and B5700 all have 12-bit syllables. As you noted, the formats are different in word mode and character mode. There is no tag. The cited text^{[1]: 5-1–5-2, Word Mode} for Operand Call and Descriptor Call is oversimplified; the ten bit relative address in subroutine mode^{[1]: 3-8, Subroutine addressing [2]: 5-4, Relative Addressing Table} is more complicated.

B590, B6x00, B7x00, etc., have 16-bit syllables; there is a tag. --Shmuel (Seymour J.) Metz Username:Chatul (talk) 14:56, 3 August 2022 (UTC)

@Guy Harris: A recent edit changed a section header from B6500 and B7500 to B65000; I believe that the B7500 was announced concurrently with the B6500 and thus was not a successor. If that is correct then B7500 should be included in the header. -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 03:00, 16 April 2023 (UTC)

I tried finding some stuff about the 7500 online, but didn't find anything indicating the dates when the 6500 and 7500 were either announced or introduced. Apparently there was a Burroughs B6500/B7500 Information Processing Systems Characteristics Manual dated 1967.

George Gray's "Burroughs Third-Generation Computers" doesn't mention the 7500, although it does mention both the B6700 and B7700, saying of the B7700 that "The 6500 and 6700 had been designed and produced in California, but now the east coast group also became involved.", mentioning the 7700 in the next paragraph. Was the 7500 announced, but never released? Was it delayed enough that it was contemporary with the 6700, so they decided to renumber it (and possibly add features added in the 6700)? Gray and Ronald Q. Smith's book Unisys Computers: An Introductory History, ISBN 9781257134090 (available in hardcover from lulu.com, although Wikipedia won't let me link there, and in bits from Apple), has a chapter "Burroughs Third-Generation Computers" that appears to be based on his article and some other sources, including Jack Allweiss' site; it has a diagram of the "Family Tree of Burroughs Computers 1962-1982" that shows no B7500, just a 7700 branching off from the B6500, along with a B6700 descended from the B6500, both with dates of 1971.

Allweiss's B5900 story mentions no B7500, just a B7700, on pages such as "The Soul of the new Machines" and "Evolution of Burroughs Stack Architecture – Mainframe Computers".

For now, given all that, I'll change the section header to "B6500, B6700/B7700, and successors" for now. Further information on the history of those systems would be interesting for several reasons:

• What was the rationale for a new large-scale descriptor-based/tagged-architecture stack machine incompatible with the B5xxx series? Binary compatibility either 1) not being recognized by Burroughs as being as important as it was to IBM or 2) being considered less important as everything was written in (somewhat) machine-independent higher-level languages?
• What was the history of the B7{5,7}00? Did they start out with the idea of two independent-but-binary-compatible (with the possible exception of some lower-level machine details that matter only to lower levels of the MCP) machines with different prices and performance levels, running the same OS and compilers, those being the B6500 and B7500? If so, was a B7500 ever released? If not, was it delayed enough that the B6700 was due to come out, and renumbered B7700 (and updated to support added features such as vector mode)? I assume it wasn't started after the B6500 came out, given the B6500/B7500 manual mentioned above; I suspect it was either dropped or was delayed and turned into the B7700.
• What were the instruction set differences between the machines, and what was the history of "E-mode"? Did it originate with the B5900 and, in the process of the B5900 design, get used in the design of machines earlier in the pipeline but not yet released, or was the B5900 the first machine released with that version of the ISA?
• What was the history all of those machines, up to the last Unisys machines that implemented the ISA in hardware/firmware rather than in a binary-to-binary translator generating 64-bit x86 code (I've seen stuff about Unisys doing that for the 1100/2200 machines, with LLVM as the back end for the translator, and suspect something similar was done for the Burroughs stack machines)? Guy Harris (talk) 08:10, 16 April 2023 (UTC)

Bitsavers has a reprint^[1] of Burroughs' B6500/7500 Stack Mechanism from AFIPS Conference Proceedings Volume 32, 1968, which uses the term B6500/B7500 to refer to both machines.

I believe that the incompatibility between the B5000 line and the B6500/B7500 line is because Burroughs identified deficiencies in the B5000 that they attempted to correct with a total redesign of the architecture -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 18:34, 17 April 2023 (UTC)

Was that paper written and published before the 6500/7500 came out, under the assumption that they'd be released at the same time? If so, did the B7500 come out? Guy Harris (talk) 18:41, 17 April 2023 (UTC)

I found this^[2] in one of the existing references. So announced but not shipped. --Shmuel (Seymour J.) Metz Username:Chatul (talk) 15:35, 18 April 2023 (UTC)