VAX

DEC VAX

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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