VAX | 1977

VAX

 VAX is a series of computers developed by Digital Equipment Corporation (DEC) featuring a 32-bit instruction set architecture (ISA) and virtual memory. The VAX-11/780 was first introduced on October 25, 1977, and was the first model to implement the VAX ISA, becoming a popular and influential computer. The VAX family was a significant success for DEC, with the last models released in the early 1990s. VAX was succeeded by DEC Alpha, which included several features to facilitate porting from VAX.

VAX was designed as a successor to the 16-bit PDP-11, providing backward compatibility with the PDP-11 while expanding memory to a full 32-bit implementation and adding demand-paged virtual memory. The name VAX refers to the concept of Virtual Address eXtension, allowing the use of this new memory while maintaining compatibility with unmodified user-mode PDP-11 code. The VAX ISA is considered a complex instruction set computer (CISC) design.

DEC quickly dropped the −11 branding as PDP-11 compatibility was no longer a major concern. The VAX line expanded to high-end mainframes like the VAX 9000 and workstation-scale systems like the VAXstation series. The VAX family ultimately included ten distinct designs and over 100 individual models, all of which were compatible with each other and typically ran the VAX/VMS operating system.

VAX is recognized as a quintessential CISC ISA due to its large number of assembly language programmer-friendly addressing modes and machine instructions, highly orthogonal instruction set architecture, and instructions for complex operations such as queue insertion or deletion, number formatting, and polynomial evaluation.

The name VAX is an acronym for Virtual Address eXtension, as VAX is seen as a 32-bit extension of the older 16-bit PDP-11. Early versions of the VAX processor implemented a "compatibility mode" that emulated many of the PDP-11's instructions, while later versions offloaded the compatibility mode and some less-used CISC instructions to emulation in the operating system software.

The VAX instruction set was designed to be powerful and orthogonal, and at the time, many programs were written in assembly language, making a "programmer-friendly" instruction set important. Over time, as more programs were written in high-level programming languages, the instruction set became less visible, and the only ones concerned about it were compiler writers.

One unusual aspect of the VAX instruction set is the presence of register masks at the start of each subprogram. These are arbitrary bit patterns that specify which registers are to be preserved when control is passed to the subprogram. In most architectures, it is up to the compiler to generate instructions to save the necessary data, typically using the call stack for temporary storage. On the VAX, with 16 registers, this might require 16 instructions to save the data and another 16 to restore it. Using the mask allows a single 16-bit value to perform the same operations internally in hardware, saving time and memory.

Since register masks are a form of data embedded within the executable code, they can complicate linear parsing of the machine code. This can complicate optimization techniques applied to machine code.

The native operating system for VAX is Digital's VAX/VMS, which was renamed OpenVMS in 1991 or early 1992 when it was ported to Alpha. The VAX architecture and VMS operating system were engineered concurrently to maximize their advantages, as was the initial implementation of the VAXcluster facility.

In the 1980s, a hypervisor for the VAX architecture named VMM (Virtual Machine Monitor) was developed, allowing multiple isolated instances of VMS and ULTRIX to run on the same hardware. VMM was designed to achieve TCSEC A1 compliance and was operational on VAX 8000 series hardware by the late 1980s, but it was abandoned before release to customers.

Other VAX operating systems included various releases of Berkeley Software Distribution (BSD) UNIX, Ultrix-32, VAXELN, and Xinu. More recently, NetBSD and OpenBSD have supported various VAX models, and efforts have been made to port Linux to the VAX architecture. OpenBSD discontinued support for the architecture in September 2016.

The VAX-11/780 was the first VAX model introduced at the Digital Equipment Corporation's Annual Meeting of Shareholders on October 25, 1977, with Bill Strecker responsible for the architecture. Subsequently, various models with different prices, performance levels, and capacities were developed, and VAX superminicomputers were very popular in the early 1980s.

For a while, the VAX-11/780 was used as a standard in CPU benchmarks, initially described as a one-MIPS machine. The actual number of instructions executed in one second was about 500,000, leading to complaints of marketing exaggeration. This resulted in the definition of a "VAX MIPS," where a computer performing at 27 VAX MIPS would run the same program roughly 27 times faster than the VAX-11/780.

Within the Digital community, the term VUP (VAX Unit of Performance) was more commonly used, as MIPS do not compare well across different architectures. The related term cluster VUPs was informally used to describe the aggregate performance of a VAXcluster. The VAX-11/780 included a subordinate stand-alone LSI-11 computer that performed microcode load, booting, and diagnostic functions for the parent computer, which was dropped from subsequent VAX models.

VAX went through many different implementations. The original VAX 11/780 was implemented in TTL and filled a four-by-five-foot cabinet with a single CPU. Throughout the 1980s, the high-end of the family was continually improved using ever-faster discrete components, an evolution that ended with the introduction of the VAX 9000 in October 1989. This design proved too complex and expensive and was ultimately abandoned not long after introduction. CPU implementations that consisted of multiple emitter-coupled logic (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. The VAX 11-730 and 725 low-end machines were built using AMD Am2901 bit-slice components for the ALU.

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 some of the more complex VAX instructions (such as the packed decimal and related opcodes) into emulation software. This 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.

A full VLSI (microprocessor) implementation of the MicroVAX architecture 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 that 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.

Further VLSI VAX processors followed in the form of the V-11, CVAX, CVAX SOC ("System On Chip," a single-chip CVAX), Rigel, Mariah, and NVAX implementations. The VAX microprocessors extended the architecture to inexpensive workstations and later also supplanted the high-end VAX models. This wide range of platforms (from mainframe to workstation) using one architecture was unique in the computer industry at that time. Various graphics were etched onto the CVAX microprocessor die. The phrase "CVAX... when you care enough to steal the very best" was etched in broken Russian as a play on a Hallmark Cards slogan, intended as a message to Soviet engineers who were known to be both purloining DEC computers for military applications and reverse engineering their chip design. By the late 1980s, the VAX microprocessors had grown in power to be competitive with discrete designs. This led to the abandonment of the 8000 and 9000 series and their replacement by Rigel-powered models of the VAX 6000, and later by NVAX-powered VAX 7000 systems.

In DEC's product offerings, the VAX architecture was eventually superseded by RISC technology. In 1989, DEC introduced a range of workstations and servers that ran Ultrix, the DECstation and DECsystem respectively, using processors from MIPS Computer Systems. In 1992, DEC introduced their own RISC instruction set architecture, the Alpha AXP (later renamed Alpha), and their own Alpha-based microprocessor, the DECchip 21064, a high-performance 64-bit design capable of running OpenVMS.

In August 2000, Compaq announced that the remaining VAX models would be discontinued by the end of the year, but old systems remain in widespread use. VMS is now developed by VMS Software Incorporated, albeit only for the Alpha, HPE Integrity, and x86-64 platforms.