DEC Prism - Prism

Prism

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 1985-1986 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. The PRISM and MIPS also lack the register windows that were a hallmark of the "other" design, Berkeley RISC/SPARC.

The PRISM design was notable for several aspects of its instruction set, however. Notably, PRISM included Epicode (extended processor instruction code), which defined a number of "special" instructions intended to offer the operating system a stable ABI across multiple implementations. Epicode was given its own set of 22 32-bit registers to use. A set of vector processing instructions were later added as well, supported by an additional sixteen 64-bit vector registers that could be used in a variety of ways.

Two versions of the system were planned, DECwest worked on a "high-end" ECL implementation known as Crystal, while the Semiconductor Advanced Development team worked on MicroPRISM, a CMOS version. MicroPRISM was finished first and was sent for test fabrication in April 1988. Additionally, Cutler led development on a new microkernel-based operating system code-named Mica, which was to offer Unix-like and VMS-like "personalities" on top of a common substrate of services.