PRISM (Parallel Reduced Instruction Set Machine) was a 32-bit RISC instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). It was the outcome of a number of DEC research projects from the 1982–1985 time-frame, and the project was subject to continually changing requirements and planned uses that delayed its introduction. This process eventually decided to use the design for a new line of Unix workstations. The arithmetic logic unit (ALU) of the microPrism version had completed design in April 1988 and samples were fabricated, but the design of other components like the floating point unit (FPU) and memory management unit (MMU) were still not complete in the summer when DEC management decided to cancel the project in favor of MIPS-based systems. An operating system codenamed MICA was developed for the PRISM architecture, which would have served as a replacement for both VAX/VMS and ULTRIX on PRISM.
PRISM's cancellation had significant effects within DEC. Many of the team members left the company over the next year, notably Dave Cutler who moved to Microsoft and led the development of Windows NT. The MIPS-based workstations were moderately successful among DEC's existing Ultrix users but had little success competing against companies like Sun Microsystems. Meanwhile, DEC's cash-cow VAX line grew increasingly less performant as new RISC designs outperformed even the top-of-the-line VAX 9000. As the company explored the future of the VAX they concluded that a PRISM-like processor with a few additional changes could address all of these markets. Starting where PRISM left off, the DEC Alpha program started in 1989.
History
Background
Introduced in 1977, the VAX was a runaway success for DEC, cementing its place as the world's #2 computer vendor behind IBM. The VAX was noted for its rich instruction set architecture (ISA), which was implemented in complex microcode. The VMS operating system was layered on top of this ISA, which drove it to have certain requirements for interrupt handling and the memory model used for memory paging. By the early 1980s, VAX systems had become "the computing hub of many technology-driven companies, sending spokes of RS-232 cables out to a rim of VT-100 terminals that kept the science and engineering departments rolling."
This happy situation was upset by the relentless improvement of semiconductor manufacturing as encoded by Moore's Law; by the early 1980s there were a number of capable 32-bit single-chip microprocessors with performance similar to early VAX machines yet able to fit into a desktop pizza box form factor. Companies like Sun Microsystems introduced Motorola 68000 series-based Unix workstations that could replace a huge multi-user VAX machine with one that provided even more performance but was inexpensive enough to be purchased for every user that required one. While DEC's own microprocessor teams were introducing a series of VAX implementations at lower price-points, the price-performance ratio of their systems continued to be eroded. By the later half of the 1980s, DEC found itself being locked out of the technical market.
RISC
During the 1970s, IBM had been carrying out studies of the performance of their computer systems and found, to their surprise, that 80% of the computer's time was spent performing only five operations. The hundreds of other instructions in their ISAs, implemented using microcode, went almost entirely unused. The presence of the microcode introduced a delay when the instructions were decoded, so even when one called one of those five instructions directly, it ran slower than it could if there was no microcode. This led to the IBM 801 design, the first modern RISC processor.
Around the same time, in 1979, Dave Patterson was sent on a sabbatical from University of California, Berkeley to help DEC's west-coast team improve the VAX microcode. Patterson was struck by the complexity of the coding process and concluded it was untenable. He first wrote a paper on ways to improve microcoding, but later changed his mind and decided microcode itself was the problem. He soon started the Berkeley RISC project. The emergence of RISC sparked off a long-running debate within the computer industry about its merits; when Patterson first outlined his arguments for the concept in 1980, a dismissive dissenting opinion was published by DEC.