DEC Alpha: Encyclopedia - DEC Alpha

DEC Alpha

The DEC Alpha, also known as the Alpha AXP, is a 64-bit RISC microprocessor originally developed and fabricated by Digital Equipment Corp. (DEC), which used it in its own line of workstations and servers. Designed as a successor to the VAX line of computers, it supported the VMS operating system, as well as Digital UNIX. Later open source operating systems also ran on the Alpha, notably Linux and BSD UNIX flavours. Microsoft supported the processor until Windows NT 4.0 SP6 but did not extend Alpha support beyond release candidate 2 of Windows 2000.

DEC Alpha - History

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 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.

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".

DEC Alpha - Versions

The first version, the Alpha 21064 was introduced in 1992 running at 192MHz, a slight shrink of the die (21064) ran at 200Mhz 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 micro-architecture was fairly similar to other RISC chips. In comparison, the less expensive Intel Pentium ran at 66MHz when it was launched the following spring.

The Alpha 21164 became available in 1995 at a processor frequency of 333MHz. In July 1996 the line was speed bumped to 500MHz, in March 1998 to 666MHz, and in May 2000 the 21264 was released at 731MHz. 1GHz and faster pieces were announced in 2001 (the 21364 or EV7), and have been available since 2003 at 1.1GHz+. Around 500,000 Alpha based systems were sold to end-2000.

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.

Compaq announced that computers using Alpha would be phased out by 2004 in favour of Intel's Itanium. HP, new owner of Compaq, announced that development for the Alpha series would continue for a few more years, including the release of the EV7z chip (EV79 and EV8 are both cancelled), but that this will be the final iteration of the chip. The IA-64 is supposed to replace this series. HP will continue maintaining and selling AlphaServers with OpenVMS and Tru64 until 2006, and has extended support to 2011.

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

On August 16, 2004 HP announced that they are releasing the 1.3GHz EV7z, that has now been released and no more releases are planned.

DEC Alpha - Model history

ISA extensions:

• R: ?
• B: BWX, instructions to read 8- and 16-bit words from memory
• M: MVI, "multimedia" instructions
• F: FIX, instructions to move data between integer and floating point registers and for square root
• C: CIX, instructions for counting and finding bits
• T: Support for prefetch with modify intent to improve the performance of the first attempt to acquire a lock

Adapted from the Wikipedia article "DEC Alpha", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki