17 Facts About VAX


VAX is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century.

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The VAX family was a huge success for DEC - over 100 models were introduced over the lifetime of the design, with the last members arriving in the early 1990s.

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VAX was designed as a successor to the 16-bit PDP-11, one of the most successful minicomputers in history with approximately 600, 000 examples sold.

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The name VAX refers to its Virtual Address eXtension concept that allowed programs to make use of this newly available memory while still being compatible with unmodified user mode PDP-11 code.

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The name "VAX-11", used on early models, was chosen to highlight this capability.

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The VAX ISA is considered a complex instruction set computer design.

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VAX has been perceived as the quintessential CISC ISA, with its very large number of assembly language programmer-friendly addressing modes and machine instructions, highly orthogonal instruction set architecture, and instructions for complex operations such as queue insertion or deletion, number formatting, and polynomial evaluation.

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Name "VAX" originated as an acronym for Virtual Address eXtension, both because the VAX was seen as a 32-bit extension of the older 16-bit PDP-11 and because it was an early adopter of virtual memory to manage this larger address space.

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Early versions of the VAX processor implement a "compatibility mode" that emulates many of the PDP-11's instructions, giving it the 11 in VAX-11 to highlight this compatibility.

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VAX instruction set was designed to be powerful and orthogonal.

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One unusual aspect of the VAX instruction set is the presence of register masks at the start of each subprogram.

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MicroVAX I represented a major transition within the VAX family.

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Full VLSI implementation of the MicroVAX architecture arrived with the MicroVAX II's 78032 (or DC333) CPU and 78132 (DC335) FPU.

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The 78032 was the first microprocessor with an on-board memory management unit The MicroVAX II was based on a single, quad-sized processor board which carried the processor chips and ran the MicroVMS or Ultrix-32 operating systems.

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The MicroVAX II was succeeded by many further MicroVAX models with much improved performance and memory.

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The VAX microprocessors extended the architecture to inexpensive workstations and later supplanted the high-end VAX models.

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In DEC's product offerings, the VAX architecture was eventually superseded by RISC technology.

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