In the late 1980s, a group of Digital Electronics Corporation engineers began the design and implementation of the DEC Alpha processor. The first workstation to be based on the AXP architecture was years ahead of the desktop PCs of its time, both in terms of performance and elegance of design. Over the following decade, the research and development teams at DEC and later Compaq incrementally improved the performance, the fabrication and production mechanisms, and the compilers for the Alpha to keep it competitive with the latest products from Intel, AMD, and Sun.

Philosophy and Performance

The Alpha is a fairly typical 64-bit RISC architecture, with fixed instruction length, five instruction formats, and a seven-stage split integer/floating-point pipeline. It was used in a long series of DEC workstations starting in 1992, continuing until Digital was acquired by Compaq in 1998. Compaq then rebranded the AlphaStation division in its own name, and is still selling AlphaServers today.

What made the Alpha somewhat unique was its blistering performance and elegant architecture. DEC's main reason for designing its own processor (as opposed to using the MIPS R2000 family) was to create a future-proof architecture, with no obvious limiting design choices. For example, the Alpha's interlocked pipeline design was primarily a response to the vastly increased complexity moving from the MIPS R2000 to the higher performance R8000 and R10000 processors.

Alpha was also designed for an eventual thousandfold increase in performance over its predicted 25-year lifetime: a ten-fold increase in each of clock speed, superscalar execution, and multiprocessing would lead to an immensely powerful processor. Sadly, the Alpha project's roadmap was cut short when the DEC engineering teams were bought out by Intel, and these goals were never quite reached.


The first Alpha processor was the EV-4 21064, released at 150MHz in November 1992. This was released in the form of the DEC 3000 Model 500 AXP workstation, which also included 512KB of off-chip level 2 cache, 32MB of RAM, and a 1GB SCSI hard drive, and was a steal at only $38,995. While the machine was definitely powerful, it had difficulty with memory sub-system bandwidth: cacheable processor-intensive applications would outperform a 200MHz Pentium Pro, while memory-dependent algorithms would perform little better than a standard 100MHz Pentium.

Succeeding the 21064 was the EV-5 21164, introduced in January 1995. Initially clocked at 266MHz, and eventually at up to 767MHz, its new 0.35µm fabrication alone increased performance over its predecessor dramatically. However, the most important change was in the memory system: this was the only Alpha to use a three-tier cache heirarchy, with the first two levels on-chip. Previously, the level two cache was on a daughter board or the motherboard, and the high L1 to L2 cache latency was one of the major bottlenecks of the 21064-powered workstations.

To fit on chip, and have acceptable latency performance, both caches had to be small by modern standards: the 21164 had 8KB each for L1 instruction and data cache, and a 96KB L2 cache. Even for 1995, this cache size is relatively small, especially for the high-end workstation market at which the Alpha was aimed: to compensate, an off-chip L3 cache was required.

Another change in cache structure came in 1998 with the release of the EV-6 21264, the latest Alpha processor: the layer 2 cache was again moved off-chip, allowing it to be of a more reasonable size. In fact, the L2 cache of the 21264 was as large as the L3 cache of the 21164, so the cache heirarchy again became two-tier. This change was possible due to the improved performance and larger 128KB size of the on-chip L1. Another performance boost came from the new fabrication techniques, which pushed the launch 21264's 500MHz clock speed up to an eventual 1GHz in 1999.

The last Alpha processor ever likely to be produced is the EV-7 21364, introduced in 2001. This revision is again centered around memory sub-system modifications, such as the introduction of an on-chip Rambus controller, and an immense 1.7MB 7-way set associative cache. In fact, the 21364 is based on the 21264's core, with the only changes being support circuitry and fabrication size: at 0.18µm, it clocks up to 1.25GHz.

Planned and partially designed by Digital, but never reaching the production stage, is the EV-8 21464. This highly ambitious design was the key to the 10x speed improvement through multiprocessing in the Alpha roadmap: it was designed to have Transputer-like high-speed processor interconnects, and was intended to eventually be produced in an eight-issue or ten-issue CPU core. The takeover by Compaq and the engineer buyout by Intel killed the 21464, and it is sadly unlikely to ever see the light of day.


Great Microprocessors of the Past and Present, John Bayko,
Computer Organization & Design, Patterson and Hennessey, Morgan Kaufman
AlphaServer Profile, Digital Equipment Corporation

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