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B5000 instruction set
Syllable repertoire for B5000, B5500 and B5700 From Wikipedia, the free encyclopedia
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The Burroughs B5000 was an early stack machine (a type of computer that uses stacks instead of general-purpose registers) and one of the first computers to implement segmented virtual memory (dividing memory into logic segments, such code and data, that allow for better isolation and more flexible allocation). Introduced in the early 1960s, it was designed to support high-level programming languages through architectural features uncommon at the time.
The B5000 instruction set applied to the B5000, B5500, and B5700 models. It was not compatible with the later B6500, B7500, or B8500 series, which introduced a different architecture.
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Technical characteristics
Instruction streams on a B5000 contain 12-bit syllables, four to a word. The architecture has two modes, Word Mode[1][2] and Character Mode,[3][4] and each has a separate repertoire of syllables. A processor may be either Control State or Normal State, and certain syllables are only permissible in Control State. The architecture does not provide for addressing registers or storage directly; all references are through the 1024 word Program Reference Table (PRT), current code segment, marked locations within the stack or to the A and B registers holding the top two locations on the stack. Burroughs numbers bits in a syllable from 0 (high bit) to 11 (low bit) and in a word from 0 (high bit) to 47 (low bit).
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Word Mode
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In Word Mode, there are four types of syllables.
The interpretation of the 10-bit relative address in Operand Call and Descriptor Call depends on the setting of several processor flags. For main programs (SALF off) it is always an offset into the Program Reference Table (PRT).
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Character Mode
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