Alpha 21464

The Alpha 21464 is an unfinished microprocessor that implements the Alpha instruction set architecture (ISA) developed by Digital Equipment Corporation and later by Compaq after it acquired Digital. The microprocessor was also known as EV8 or Araña, the latter being its code-name. Slated for a 2004 release, it was canceled on 25 June 2001 when Compaq announced that Alpha would be phased out in favor of Itanium by 2004. When it was canceled, the Alpha 21464 was at a late stage of development but had not been taped out.

The 21464's origins began in the mid-1990s when computer scientist Joel Emer was inspired by Dean Tullsen's research into simultaneous multithreading (SMT) at the University of Washington. Emer had researched the technology in the late 1990s and began to promote it once he was convinced of its value. Compaq made the announcement that the next Alpha microprocessor would use SMT in October 1999 at Microprocessor Forum 1999. At that time, it was expected that systems using the Alpha 21464 would ship in 2003.

The microprocessor was an eight-issue superscalar design with out-of-order execution, four-way SMT and a deep pipeline. It fetches 16 instructions from a 64 KB two-way set-associative instruction cache. The branch predictor then selected the "good" instructions and entered them into a collapsing buffer. (This allowed for a fetch bandwidth of up to 16 instructions per cycle, depending on the taken branch density.) The front-end had significantly more stages than previous Alpha implementation and as a result, the 21464 had a significant minimum branch misprediction penalty of 14 cycles. The microprocessor used an advanced branch prediction algorithm to minimize these costly penalties.

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