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.

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.

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.

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.

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.

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.

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.

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.

VAX instruction set was designed to be powerful and orthogonal.

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

MicroVAX I represented a major transition within the VAX family.

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

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.

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

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

In DEC's product offerings, the VAX architecture was eventually superseded by RISC technology.