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VAX

DEC VAX
VAX 11-780 intero.jpg
VAX-11/780
Manufacturer Digital Equipment Corporation
Byte size 8 bits (octet)
Address bus size 32 bits
Peripheral bus Unibus, Massbus, Q-Bus, XMI, VAXBI
Architecture CISC, virtual memory
Operating systems VAX/VMS, Ultrix, BSD UNIX, VAXELN
DEC VAX registers
31 . . . 23 . . . 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 (bit position)
General registers
R0 Register 0
R1 Register 1
R2 Register 2
R3 Register 3
R4 Register 4
R5 Register 5
R6 Register 6
R7 Register 7
R8 Register 8
R9 Register 9
R10 Register 10
R11 Register 11
R12 / AP Register 12 / Argument Pointer
R13 / FP Register 13 / Frame Pointer
R14 / SP Register 14 / Stack Pointer
R15 / PC Register 15 / Program Counter
Status flags
N Z V C Condition Code Register
VAX
Designer Digital Equipment Corporation
Bits 32 bits
Introduced 1977
Design CISC
Type Memory-Memory
Encoding Variable (1 to 56 bytes)
Branching Condition code
Endianness Little
Extensions PDP-11 compatibility mode, VAXvector
Open No
Registers
General purpose 16
Floating point uses the GPRs

VAX is a discontinued instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC) in the mid-1970s. The VAX-11/780, introduced on October 25, 1977, was the first of a range of popular and influential computers implementing that architecture.

A 32-bit system with a CISC architecture based on DEC's earlier PDP-11, VAX ("virtual address extension") was designed to extend or replace DEC's various PDP ISAs. 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 assembly-language-programmer-friendly addressing modes and machine instructions, highly orthogonal architecture, and instructions for complex operations such as queue insertion or deletion and polynomial evaluation.

The 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 (after Prime Computer) an early adopter of virtual memory to manage this larger address space. Early versions of the VAX processor implemented a "compatibility mode" that emulated many of the PDP-11's instructions, and were in fact called VAX-11 to highlight this compatibility and that VAX-11 was an outgrowth of the PDP-11 family. Later versions offloaded the compatibility mode and some of the less used CISC instructions to emulation in the operating system software.


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