Gecko's CPU Library

Alpha architecture

Introduction: 1992

The DEC Alpha, also known as the Alpha AXP, is a 64-bit RISC microprocessor originally developed and fabricated by Digital Equipment Corporation (DEC). Designed to power successors to the VAX line of computers, 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 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 Emerald, 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.

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.

Officially, the Alpha processors were designated the DECchip 21x64 series, the "21" signifying the 21st century, and the "64" indicating 64 bits, with the middle digit corresponding to the generation of the Alpha architecture. Internally, Alpha processors were also identified by EV numbers, EV officially standing for "Extended VAX" but having an alternative humorous meaning of "Electric Vlasic".

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 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 core.

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 Alpha series was sold, along with most parts of DEC, to Compaq in 1998. Compaq, already an Intel customer, decided to phase out Alpha in favor of the forthcoming Intel IA-64 architecture, and sold all Alpha intellectual property to Intel in 2001, effectively killing the product. Hewlett-Packard purchased Compaq later that same year, continuing development of the existing product line until 2004, and promising to continue selling Alpha-based systems, largely to the existing customer base, until October 2006. In November 2006, HP announced an extension to sales until 27th April 2007.

Source: Wikipedia, the free encyclopedia.