DEC PRISM

Prism is a 32-bit RISC instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). It was the final outcome of a number of DEC research projects from the 1982–85 time-frame, and was at the point of delivering silicon in 1988 when the management canceled the project. The next year work on the Alpha started, based heavily on the Prism design.

In the period from 1982 to 1985, no fewer than four attempts were made to create a RISC chip at different DEC divisions. Titan from DEC's Western Research Laboratory (WRL) in Palo Alto, California was a high-performance ECL based design that started in 1982, intended to run Unix. SAFE (Streamlined Architecture for Fast Execution) was a 64-bit design that started the same year, designed by Alan Kotok (of Spacewar! fame) and Dave Orbits and intended to run VMS. HR-32 (Hudson, RISC, 32-bit) started in 1984 by Rich Witek and Dan Dobberpuhl at the Hudson fab, intended to be used as a co-processor in VAX machine. The same year Dave Cutler started the CASCADE project at DECwest in Bellevue, Washington.

Eventually Cutler was asked to define a single RISC project in 1985, selecting Rich Witek as the chief architect. The design started as a 64-bit chip, but was later "downsized" to 32-bits. In August 1985 the first draft of a high-level design was delivered, and work began on the detailed design. The PRISM specification was developed over a period of many months by a five-person team: Dave Cutler, Dave Orbits, Rich Witek, Dileep Bhandarkar, and Wayne Cardoza. This work was 98% done 1985–86 and was heavily supported by simulations by Pete Benoit on a large VAXcluster.

In terms of integer operations, the PRISM architecture was similar to the MIPS designs. Of the 32-bit instructions, the 6 highest and 5 lowest bits were the instruction, leaving the rest of the word for encoding either a constant or register locations. Sixty-four 32-bit registers were included, as opposed to thirty-two in the MIPS, but usage was otherwise similar. PRISM and MIPS both lack the register windows that were a hallmark of the other major RISC design, Berkeley RISC/SPARC.

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