PA-RISC: Not Dead Yet!
Throughout its existence, Hewlett Packard’s Precision Architecture RISC family has consistently been a top performer. Remarkably, PA-RISC has kept close to the front of the RISC pack (and occasionally taken the lead) over the last five years on the strength of a processor core design which is nearly as old as Intel’s P6. This core, the first 64-bit PA-RISC implementation, was first used in the 0.5 um PA-8000, which relied on large external L1 caches to keep pace with Alpha processors that were clocked more than twice as fast as its leisurely 180 MHz. It was given a makeover in the same process and became known as the PA-8200, which ran as fast as 240 MHz. Given new life as the gargantuan PA-8500 in Intel’s 0.25 um P856, this core was coupled with 1.5 Mbyte of on-chip L1 cache to act as the strongest challenger to Compaq’s sophisticated 0.35 um EV6. The PA-8500 was recently taken into the shop for a tune-up and came out as the PA-8600. Despite being made in essentially the same 0.25 um process as the PA-8500, the PA-8600 relies on critical path cleanups and fine tuning of its cache architecture to provide a 20% performance increase to stay right on the tail of the 0.25 um EV67.
Although HP is publicly committed to switching its high end product line over from PA-RISC to the IA-64 architecture it jointly designed with Intel, it appears to be in no hurry at all to rush the transition. On its public PA-RISC roadmap, HP has identified three additional designs – the PA-8700, the PA-8800, and the PA-8900. The PA-8700 is described as basically a PA-8600 shrunk to a 0.18 um copper SOI process with the on-chip caches increased in size by 50% to 2.25 Mbyte. The fact that the PA-8700 will utilize an SOI copper process immediately ruled out Intel for the role of the manufacturer. But it wouldn’t take Sherlock Holmes to notice that the PA-8700 process metal pitches identified in a recent white paper from the HP web site were identical to those of IBM’s CMOS8S process shown in Table 1. And indeed it was recently confirmed that IBM Microelectronics would manufacture the PA-8700 starting early next year.
There is little information to suggest that the PA-8800 and PA-8900 are any more than continued minor enhancements and process shrinks of the remarkably long-lived PA-8000 processor core. Even more astounding is that HP classifies the performance levels of the PA-8800 and PA-8900 as similar to the 2nd and 3rd generation IA-64 processors McKinley and Deerfield respectively. No doubt the reluctance of HP to part with its proprietary RISC processor line must cause some degree of consternation in Santa Clara. The fact that the originator of the EPIC processor design concept sees fit to spend precious resources to retain its RISC family, refreshed to at least 2004 or so, is proving an embarrassment to Intel’s attempt to position its first few IA-64 processors as the divinely appointed successor to all RISC-based computing.
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