Alpha 21064

Lore & Background

The first Alpha processor was a test chip codenamed EV3, fabricated using Digital's 1.0 μm CMOS-3 process. It lacked a floating-point unit and had only 1 KB caches, used to confirm aggressive circuit design techniques and to bring up firmware and operating systems. The production chip, EV4, was fabricated using Digital's 0.75 μm CMOS-4 process. Dirk Meyer and Edward McLellan were the micro-architects; Ed designed the issue logic while Dirk designed the other major blocks, and Jim Montanaro led the circuit implementation. The EV3 was used in the Alpha Demonstration Unit, a multiprocessor system for software development before EV4 parts were available.

The 21064 was unveiled at the 39th International Solid-State Circuits Conference in mid-February 1992 and announced on 25 February 1992 with a 150 MHz sample introduced the same day. Volume shipments began in September 1992. A 200 MHz version was introduced on 25 February 1993, with sample kits priced at $3,495; volume orders were accepted in June 1993 with shipments in August 1993. With the introduction of the Alpha 21066 and Alpha 21068 on 10 September 1993, a 166 MHz version was introduced. DEC was the only manufacturer, with no second source, and the chip was fabricated at Digital's Hudson, Massachusetts and South Queensferry, Scotland facilities.

The 21064 was mostly used in high-end computers such as workstations and servers. Users included Aspen Systems, Carrera Computers, Cray Research (which used the 150 MHz 21064 in its Cray T3D supercomputers), Digital in various DEC AXP systems, and Encore Computer. It was the highest performing microprocessor from its introduction until 1993, after IBM introduced the multi-chip POWER2, and remained the highest performing single-chip microprocessor until the 275 MHz 21064A was introduced in October 1993.

Reader's Guide

The Alpha 21064 established Digital Equipment Corporation's presence in the high-performance microprocessor market as the first commercial implementation of the Alpha ISA. Its superpipelined dual-issue superscalar design, with a seven-stage integer pipeline and ten-stage floating-point pipeline, set a new performance benchmark at its introduction. The chip's branch prediction, combining static and dynamic methods with a 2,048-entry branch history table, achieved approximately 80% accuracy for most programs. The 21064's 43-bit virtual address and 34-bit physical address enabled addressing of 8 TB of virtual memory and 16 GB of physical memory. Its write buffer improved performance by merging adjacent stores and reducing system bus utilization. The 21064 was succeeded by the Alpha 21064A in October 1993, which was in turn replaced by the Alpha 21164 in 1995. The chip's pricing strategy, with reductions over time to compete against processors like the 60 MHz Pentium, reflected its positioning in the workstation and server markets. Its use in the Cray T3D supercomputer demonstrated its capability in high-performance computing.

Did You Know?

• The 21064 was also known by its code name EV4.
• The 21064 could issue up to two instructions every clock cycle to four functional units.