VAX

Introduction
The VAX family of computers was one of the most successful series of computer systems ever developed. Its success made DEC the second-largest company in the industry. Over a 100,000 computers were sold by DEC ranging from the MicroVAX II to the VAX 9000, a number that surpassed that of the pioneering IBM System 360/370 series. The VAX design was implemented from scratch several times to capitalize on advances in technology and the changing needs of customers. Different implementations have used a variety of technologies, organizational techniques and configurations to create a broad set of systems.

The goal set out for the VAX design was an architecture that would apply to all members of the VAX family. Hardware engineers were free to build different hardware instantiations up to the specification while applications and system programmers could safely program down to it, confident that any program conforming to the specification would run on any present or future machine. The VAX moniker 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 an early adopter of virtual memory to manage the larger address space (2GB data and 2GB instruction space).

Lack of virtual address space has been the Achilles heel of most computer architectures. Not long after the PDP-11 was announced in 1969, DEC realized customers were going to demand computers with more than 64KBytes of memory; the maximum amount that can be addressed directly by a 16 bit address. The relentless progress in memory chip densities was quadrupling every three to four years. Increasing densities yields decreasing memory costs. Over the years the PDP-11 memory was extended to 256KBytres and then up to 2MBytes.

However, the virtual address of the PDP-11 remained at 64KBytes and a programmer often faced the tedious task of mapping and remapping the PDP-11s small virtual address into a much larger physical memory. The desire to build a machine with sufficient address space to satisfy customer needs for many years led to the decision for DEC to build a new 32-bit architecture. With 32 bit addressing, 4GBytes of address space was possible. The first VAX-11/780 shipped in early 198 had 256K of physical memory, built from 4K-bit dynamic memory chips. By contrast, the VAX8650, could be configured with 68MBytes of memory built using 256Kbit memory chips.

Early versions of the VAX processor implement a compatibility mode that emulated many of the PDP-11's instructions, giving it the 11 in VAX-11 to highlight this compatibility. Later versions dropped the compatibility mode and some of the less used CISC (Complex Instruction Set Computer) instructions to software emulation in the operating system software.

VAX was a CISC instruction set architecture (ISA) and line of super minis and workstations developed by the 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 the VAX ISA. Over 100 models were introduced over the lifetime of the design, with the last members arriving in the early 1990s. The VAX was succeeded by the DEC Alpha, which included several features from VAX machines to make porting from the VAX easier.

VAX was designed as a successor to the 16-bit PDP-11, one of the most successful minicomputers in history with approximately 600,000 examples sold. The system was designed to offer backward compatibility with the PDP-11 while extending the memory to a full 32-bit implementation and adding demand paged virtual memory. The name VAX refers to its Virtual Address eXtension concept that allowed programs to make use of this newly available memory while still being compatible with unmodified PDP-11 code. The name VAX-11, used on early models, was chosen to highlight this capability.

Later models in the series dropped the VAX-11 branding as PDP-11 compatibility was no longer a major concern. The line expanded to both high-end machines such as the VAX 9000 as well as to the workstation-scale systems; the VAXstation series. The VAX family ultimately contained ten distinct designs and over 100 individual models in total. All of these were compatible with each other, and ran the VAX/VMS operating system.

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, number formatting, and polynomial evaluation. It is historically one of the most studied and commented-on ISA's in computer history.

History
The first VAX model sold was the VAX-11/780, known internally as Star, which was introduced on October 25, 1977 at the Digital Equipment Corporation's Annual Meeting of Shareholders. Bill Strecker, Gordon Bell's doctoral student at Carnegie Mellon University, was responsible for the architecture. Many different models with different prices, performance levels, and capacities were subsequently created. VAX super minicomputers were very popular in the early 1980s.

The KA780 CPU was built from Schottky transistor-transistor (TTL) logic and features a 200ns cycle time (5MHz) with a 2KB cache. Memory and I/O were accessed via the Synchronous Backplane Interconnect (SBI). The ALU was built from off-the shelf 74181 circuits, rather than the more popular AMD2901 ALU used in many other minicomputers. One of the reasons cited was the lower cost.

For a while the VAX-11/780 was used as a standard in CPU benchmarks. It was initially described as a one-MIPS machine, because its performance was equivalent to an IBM System/360 that ran at one MIPS, and the System/360 implementations had previously been de-facto performance standards. The actual number of instructions executed in 1 second was about 500,000, which led to complaints of marketing exaggeration. The result was the definition of a "VAX MIPS," the speed of a VAX-11/780; a computer performing at 27 VAX MIPS would run the same program roughly 27 times faster than the VAX-11/780.

This was dropped from subsequent VAX models. Enterprising VAX-11/780 users could therefore run three different Digital Equipment Corporation operating systems: VMS on the VAX processor (from the hard drives), and either RSX-11S or RT-11 on the LSI-11 (from the single density single drive floppy disk).

Within the Digital community the term VUP (VAX Unit of Performance) was the more common term, because 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 performance of the VAX-11/780 still serves as the baseline metric in the BRL-CAD Benchmark, a performance analysis suite included in the BRL-CAD solid modeling software distribution.) The VAX-11/780 included a subordinate stand-alone LSI-11 computer that performed microcode patch load, booting, and diagnostic functions for the parent computer.

Most of the microcode actually resided in PROMs on the M8234 card within the mainframe chassis. There was a WCS patch area on the M8233 card that loaded patch microcode from the LSI-11. In time, the number of patch code grew so much that it no longer fit on the M8233 card and an updated version of the M8234 card had to be released.

Technical Implementations
The 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. CPU implementations that consisted of multiple ECL gate array or macrocell array chips included the VAX 8600 and 8800 super minis 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 VAX-11/750, code-named Comet, was a compact, lower-performance Bipolar TTL gate array–based implementation of the VAX architecture introduced in October 1980. The use of gate arrays decreased power consumption, and increased reliability as compared to the VAX-11/780.

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 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.

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 (mainframe to workstation) using one architecture was unique in the computer industry at that time. Sundry 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 copying DEC computers for military applications and reverse engineering their chip design.

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, based on processors that implemented the MIPS architecture. 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. By 2005 all manufacturing of VAX computers had ceased, but many older systems remain in widespread use.

Operating Systems
VMS is now managed by, VMS Software Incorporated although they only offer OpenVMS for Alpha systems and HPE Integrity Servers, with x86-64 support being developed, and do not offer it for VAX. The VAX architecture and OpenVMS operating system were engineered concurrently to take maximum advantage of each other, as was the initial implementation of the VAXcluster facility. Other VAX operating systems have included various releases of BSD UNIX up to 4.3BSD, Ultrix-32, VAXELN, and Xinu. More recently, NetBSD and OpenBSD have supported various VAX models and some work has been done on porting Linux to the VAX architecture.

Source: Text adapted from Wikipedia.org.

Sources:

• Wikipedia