VAX 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.
VAX 11/780 (first two cabinet sections), shown with Unibus expansion cabinet (middle cabinet section), two tape drives, two RP05 or RP06 removable pack disk drives, a DECwriter printing terminal, and a VT52 CRT terminal.
"VAX" is originally 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 the fact 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.
The VAX went through many different implementations. The original VAX was implemented in TTL and filled more than one rack for a single CPU. CPU implementations that consisted of multiple ECL gate array or macrocell array chips included the VAX 8600 and 8800 superminis and finally the VAX 9000 mainframe class machines. CPU implementations that consisted of multiple MOSFET custom chips included the 8100 and 8200 class machines.
A full VLSI (microprocessor) implementation of the MicroVAX architecture then 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 machine featured 1 MB of on-board memory and a Q22-bus interface with DMA transfers. The MicroVAX II was succeeded by many further MicroVAX models with much improved performance and memory.
The MicroVAX I represented a major transition within the VAX family. At the time of its design, it was not yet possible to implement the full VAX architecture as a single VLSI chip (or even a few VLSI chips as was later done with the V-11 CPU of the VAX 8200/8300). Instead, the MicroVAX I was the first VAX implementation to move most of the complexity of the VAX instruction set into emulation software, preserving just the core instructions in hardware. This new partitioning substantially reduced the amount of microcode required and was referred to as the "MicroVAX" architecture. In the MicroVAX I, the ALU and registers were implemented as a single gate-array chip while the rest of the machine control was conventional logic.
The 1990s had downsized the computer industry; Digital was no exception. Between 1991 and 1994, the firm lost over $4 billion. At the end of the 1994 fiscal year (June), Digital reported yearly operating revenues of $13.5 billion, down 6% from 1993, an annual net loss of over $2 billion (including restructuring charges of $1.2 billion), and 85,000 employees. The firm's cost structure was higher than average for its industry. Gross margins on products had declined due to pricing and a continued shift in mix to low-end, low-margin products.
Eventually, on 26 January 1998, what remained of the company (including Digital's multivendor global services organization and customer support centers) was sold to Compaq, which was acquired by Hewlett-Packard in 2002. The remainder of Digital Semiconductor (the Alpha microprocessor group) was sold to Intel, which placed those employees back in their Hudson, Mass. office which they had vacated when the site was sold to Intel in 1997. Compaq, and later HP, continued to sell many of the former Digital products but rebranded with their own logos. For example, HP now sells what were formerly Digital's StorageWorks disk/tape products, as a result of the Compaq acquisition.
The digital.com and DEC.com domain names are now owned by Hewlett-Packard and redirect to their US website. Digital once held the Class A IP address block 126.96.36.199/8.
The Digital Federal Credit Union (DCU, now DFCU), which was chartered in 1979 for employees of DEC, is now open to essentially everyone. DFCU has over 700 different sponsors, including the companies that acquired pieces of DEC.
In August 2000, Compaq announced that the remaining VAX models would be discontinued by the end of the year. By 2005 all manufacturing of VAX computers had ceased, but old systems remain in widespread use.
The SRI CHARON-VAX and SIMH software-based VAX emulators remain available today.