DEC PRISM

DEC PRISM

Designer	Digital Equipment Corporation
Bits	32-bit
Introduced	1988 (cancelled)
Design	RISC
Successor	DEC Alpha
Registers
64× 32-bit general purpose registers 16× 64-bit vector registers

PRISM (Parallel Reduced Instruction Set Machine)^[1] 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.^[2] 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.^[3]

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.