Our report that Silicon Graphics Inc was talking up the similarities between its MIPS chip and the Alpha RISC may have been no more than pure smoke to present its acquisition of Cray Research Inc’s Alpha-based massively parallel systems technology in the best possible light (CI No 2,860). But it sure piqued some interest out there. We asked Digital Equipment Corp whether there is much that is common. It told us the Alpha architects looked at all the other RISC architectures around and grabbed any ideas that seemed useful. They saw the features that were starting to cause pain in other implementations and it made them avoid things like register windows (Sparc), flag registers (everyone, to its knowledge), and branch delay slots (Sparc and MIPS). All of these apparently get very painful once you try to build superscalar (multiple-issue-per-clock) processors. Having said that, it admits the stuff that iAPX-86 cloners have done to increase performance vindicates the statement that the inherent difference between any of the architectures is smaller than the difference a good design team could achieve for a given generation of implem entations. DEC says that if Alpha, Sparc and MIPS are compared feature-for- feature then Alpha probably looks more like MIPS than Sparc, but isn’t sure that there is any deeper similarity than that. DEC has made several small changes to Alpha’s bus interface specifically to satisfy requirements that Cray had.
