VAXen:

The VAX has been perceived as the quintessential CISC processing architecture, with its very large number of addressing modes and machine instructions, including instructions for such complex operations as queue insertion/deletion and polynomial evaluation.^{[citation needed]}

Name

"VAX" was 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 plural form of VAX is usually VAXes, but VAXen is also heard.

Operating systems

The "native" VAX operating system is DEC's VAX/VMS (renamed to OpenVMS in 1991 or 1992 when it was ported to DEC Alpha, "branded" by the X/Open consortium, and modified to comply with POSIX standards^[1]^{[citation needed]}). The VAX architecture and VMS 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 and VAXELN. More recently, NetBSD and OpenBSD support various VAX models and some work has been done on porting Linux to the VAX architecture.

History

The first VAX model sold was the VAX-11/780, which was introduced on October 25, 1977 at the Digital Equipment Corporation's Annual Meeting of Shareholders.^[2] Bill Strecker, C. Gordon Bell's doctoral student at Carnegie-Mellon University, was responsible for the architecture.^[3] Many different models with different prices, performance levels, and capacities were subsequently created. VAX superminis were very popular in the early 1980s.

For a while the VAX-11/780 was used as a baseline in CPU benchmarks because its speed was about one MIPS. Ironically enough, though, the actual number of instructions executed in 1 second was about 500,000. One VAX MIPS was the speed of a VAX-11/780; a computer performing at 27 VAX MIPS would run the same program roughly 27 times as fast as the VAX-11/780. 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 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 8600, 8800 superminis and finally the 9000 mainframe class machines. CPU implementations that consisted of multiple MOSFET custom chips included the 8100 and 8200 class machines.

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.

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^[4] 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, 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 both purloining DEC computers for military applications, along with reverse engineering their chip design.^[5] ^[6]

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 capable of running OpenVMS.

In August 2000, Compaq announced that the remaining VAX models would be discontinued by the end of the year.^[7] By 2005 all manufacturing of VAX computers had ceased, but old systems remain in widespread use.

Processor architecture

Virtual Memory Map

The VAX virtual memory is divided into four sections, each of which is one gigabyte^[8] in size:

For VMS, P0 was used for user process space, P1 for process stack, S0 for the operating system, and S1 was reserved.

Privilege Modes

Processor Status Register

VAX models

Listed in roughly chronological order. The codenames used during development within Digital Equipment Corporation are shown in italic. VAX systems can be broadly classified into those with non-VLSI processors and those with VLSI processors with the MicroVAX-I being a transitional design:

Non-VLSI VAXen

A transitional VAX

VLSI VAXen

Miscellaneous

Clones

A number of clones of VAX models, both authorized and unauthorized, were produced. Examples include:

References

^ ^a ^b Rainville, Jim & Howard, Karen, eds. (1997), VAX/VMS at 20, Digital Equipment Corporation, <http://h71000.www7.hp.com/openvms/20th/>. Retrieved on 13 October 2007
^ "VAX 11/780, The First VAX System (October 1977)".
^ Slater, Robert (1987), Portraits in Silicon, MIT Press, p. 213, <http://books.google.com/books?id=aWTtMyYmKhUC&dq=%22Bill+Strecker%22+vax+designer&source=gbs_summary_s&cad=0>
^ The Computer History Simulation Project: MicroVAX II (1985)
^ micro.magnet.fsu.edu, Steal the best, retrieved 30 January 2008. The Russian phrase was: СВАКС... Когда вы забатите довольно воровать настоящий лучший
^ The Computer History Simulation Project: CVAX (1987), retrieved 30 January 2008
^ "VAX Systems: A letter from Jesse Lipcon".
^ In the context of addressing, 1 GB is equal to 2³⁰ bytes
^ VAX timeline, Hewlett Packard website
^ DIGITAL Computing Timeline
^ The Computer History Simulation Project: Rigel (1989)
^ Henry M. Levy (January 1984). "VAXstation: A General-Purpose Raster Graphics Architecture". ACM Transactions on Graphics (TOG) 3: 70–83. ACM. doi:10.1145/357332.357336.
^ Bell, Gordon (1986). "Toward a History of (Personal) Workstations". Proc ACM Conference on the History of Personal Workstations: 1-17, ACM. Retrieved on 2007-10-13.
^ "RAL Informatics Report 1984-85". Retrieved on 2007-10-15.
^ "The TPA story". Retrieved on 2007-10-15.

VAXen

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