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VAX (Virtual Address eXtension) was an instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC) in the mid-1970s. A 32-bit complex instruction set computer (CISC) ISA, it was designed to extend or replace DEC's various Programmed Data Processor (PDP) ISAs. The VAX name was also used by DEC for a family of computer systems based on this processor architecture.

The VAX architecture's primary features were virtual addressing (for example demand paged virtual memory) and its orthogonal instruction set. VAX has been perceived as the quintessential CISC ISA, with its very large number of programmer-friendly addressing modes and machine instructions, highly orthogonal architecture, and instructions for complex operations such as queue insertion or deletion and polynomial evaluation.

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