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Alpha AXP EV68CB processor
Derived fromSystem Component, Runnable
Connects toTyphoon chipset
List of all devices

Functional Description

The DEC Alpha, also known as the Alpha AXP, is a 64-bit RISC microprocessor originally developed and fabricated by Digital Equipment Corporation (DEC). It was designed to replace the 32-bit VAX processor. It was used in a variety of DEC workstations and servers, eventually forming the basis for almost all of their mid-to-upper-scale lineup. Several third-party vendors also produced Alpha systems, as well as PC compatible form factor motherboards.

Alpha supports both the OpenVMS operating system and Tru64 UNIX (previously known as DEC OSF/1 AXP or Digital UNIX). Open source operating systems also run on the Alpha, notably Linux and BSD UNIX flavors (FreeBSD support ended as of 7.0). Microsoft supported the processor in Windows NT until Windows NT 4.0 SP6 but did not extend Alpha support beyond RC1 of Windows 2000.

The Specific CPU emulated is an Alpha AXP EV68CB[1][2] (DEC Chip 21264), which complies with the Alpha Architecture standard[3][4]


Alpha was born out of an earlier RISC project named PRISM, itself the final product of several earlier projects. DEC had been marketing the DECstation line of workstations based on the MIPS architecture, and unsurprisingly PRISM shared many features with MIPS. Among the differences between PRISM and MIPS, however, was that PRISM supported a user-programmable microcode known as Epicode. PRISM had been designed with the intent of releasing a new operating system along with it, known as Mica, which would allow it to run "native" programs at full speed while also supporting Digital's existing VMS programs from the VAX after minor conversion. DEC management doubted the need to produce a new computer architecture to replace their existing VAX and DECstation lines, and eventually killed the PRISM project in 1988.

By the time of cancellation, however, second-generation RISC chips (such as the newer SPARC architecture), were offering much better price/performance ratios than the VAX lineup. It was clear a third generation would completely outperform the VAX in all ways, not just on cost. Another study was started to see if a new RISC architecture could be defined that could directly support the VMS operating system. The new design used most of the basic PRISM concepts, but was re-tuned to allow VMS and VMS programs to run at reasonable speed with no conversion at all. The decision was also made to upgrade the design to a full 64-bit implementation from PRISM's 32-bit, a conversion all of the major RISC vendors were undertaking. Eventually that new architecture became Alpha. The Alpha instruction set architects were Dick Sites and Rich Witek. The PRISM's Epicode was developed into the Alpha's PALcode, providing an abstracted interface to platform- and processor implementation-specific features.

A persistent report attributed to DEC insiders suggests the choice of the AXP tag for the processor was made by DEC's legal department, which was still smarting from the VAX trademark fiasco. After a lengthy search the tag "AXP" was found to be entirely unencumbered. Within the computer industry, a joke got started that the acronym AXP meant "Almost Exactly PRISM".


The main contribution of Alpha to the microprocessor industry, and the main reason for its excellent performance, was not so much the architecture but rather superb implementation. At that time (as it is now), the microchip industry was dominated by automated design and layout tools. The chip designers at Digital continued pursuing sophisticated manual circuit design in order to deal with the overly complex VAX architecture. The Alpha chips showed that manual circuit design applied to a simpler, cleaner architecture allowed for much higher operating frequencies than those that were possible with the more automated design systems. These chips caused a renaissance of custom circuit design within the microprocessor design community.

The first few generations of the Alpha chips were some of the most innovative of their time. The first version, 21064 or EV4, was the first CMOS microprocessor whose operating frequency rivalled higher-powered ECL minicomputers and mainframes. The second, 21164 or EV5, was the first microprocessor to place a large secondary cache on chip. The third, 21264 or EV6, was the first microprocessor to combine both high operating frequency and the more complicated out-of-order execution microarchitecture. The 21364 or EV7 was the first high performance processor to include an Integrated Memory Controller. The unproduced EV8 would have been the first to include simultaneous multithreading, but this version was caught up in the sale to Compaq. The Tarantula research project, which most likely would have been called EV9, would have been the first processor to feature a powerful vector processor core.


At the time of its announcement, Alpha was heralded as an architecture for the next 25 years. While this was not to be, Alpha has nevertheless had a reasonably long life. The first version, the Alpha 21064 (otherwise known as the EV4) was introduced in November 1992 running at up to 192 MHz; a slight shrink of the die (the EV4S, shrunk from 0.75 µm to 0.675 µm) ran at 200 MHz a few months later. The 64-bit processor was a superpipelined and superscalar design, like other RISC designs, but nevertheless outperformed them all and DEC touted it as the world's fastest processor. Careful attention to circuit design, a hallmark of the Hudson design team, like a huge centralized clock circuitry, allowed them to run the CPU at higher speeds, even though the microarchitecture was fairly similar to other RISC chips. In comparison, the less expensive Intel Pentium ran at 66 MHz when it was launched the following spring.

The Alpha 21164 or EV5 became available in 1995 at processor frequencies of up to 333 MHz. In July 1996 the line was speed bumped to 500 MHz, in March 1998 to 666 MHz. Also in 1998 the 21264 (EV6) was released at 450 MHz, eventually reaching (in 2001 with the 21264C/EV68CB) 1.25 GHz. In 2003, the EV7 Marvel was launched, essentially an EV68 core with four 1.6 GB/s inter-processor communication links for improved multiprocessor system performance, running at 1 or 1.15 GHz. Around 500,000 Alpha based systems were sold by the end of 2000.

In 1999, the production of Alpha chips was licensed to Samsung Electronics Company. Following the purchase of Digital by Compaq the majority of the Alpha products were placed with API NetWorks, Inc. (previously Alpha Processor Inc.), a private company funded by Samsung and Compaq. In October 2001, Microway became the exclusive sales and service provider of API NetWorks' Alpha-based product line.

On June 25, 2001, Compaq announced that Alpha would be phased out by 2004 in favor of Intel's Itanium, canceled the planned EV8 chip, and sold all Alpha intellectual property to Intel. HP, new owner of Compaq later the same year, announced that development of the Alpha series would continue for a few more years, including the release of a 1.3 GHz EV7 variant called the EV7z. This would be the final iteration of Alpha, the 0.13 µm EV79 also being canceled. HP stopped selling AlphaServers with OpenVMS and Tru64 UNIX on April 27, 2007, and has promised support until at least 2012.

Ironically, in mid-2003, as the Alpha was about to be phased out, the fastest and second fastest computers (in 2002) in the United States were both implemented using Alpha processors (in the case of the former, a cluster of 4096 Alpha processors).


  1. Alpha 21264 Microprocessor Product Brief (pdf) at location 1, location 2
  2. 21264/EV68CB and 21264/EV68DC Hardware Reference Manual (pdf) at location 1, location 2
  3. Alpha Architecture Handbook (pdf) at location 1, location 2
  4. Alpha Reference Manual, Fourth Edition (pdf) at location 1

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