VAX

SPEC-1 VAX 1 SPEC-1 VAX ¹

The SPEC-1 VAX, the VAX 11/780 that was used as the benchmark for the speed of each of DEC's VAXes. Now on display at the Department of Computer Science & Engineering, University of Washington, Seattle, Washington, Photo by Joe Mabel, August 03, 2009, VAX from Wikipedia, Wikimedia Commons ↩︎

VAX, (Virtual Address Extension)
an instruction set architecture, developed by Digital Equipment Corporation (DEC) in the mid-1970s, to extend the PDP-11 architecture to a 32-bit orthogonal instruction set while adding a complete virtual memory system to the simple paging and memory protection of the PDP-11. The first computer to use a VAX CPU was the VAX-11/780, which DEC referred to as a Superminicomputer ¹. DEC's VAX strategy was formulated by Gordon Bell ².

Contents

Registers

VAX has 16 32-bit registers, R0-R15. R0-R11 general purpose, R12/AP the argument pointer, R13/FP the frame pointer, R14/SP the stack pointer, and R15/PC the program counter or instruction pointer.

Instructions

The general form of a VAX instruction ...

opcode [ operand ] [ operand ] ...

... has each component being one byte, the opcode a value in the range 0 - 255, and each operand consisting of two nibbles, the upper 4 bits specifying an addressing mode, and the lower 4 bits (usually) a register number (R0 - R15).

MicroVAX

DEC MicroVAX CPU die ³ The MicroVAX was a family of low-end minicomputers implemented the VAX instruction set architecture, first introduced in 1984.