Editing DEC PRISM (section)

{{Short description|RISC instruction set architecture}}
{{distinguish|Apollo PRISM}}
{{Infobox CPU architecture
| name        = DEC PRISM
| designer    = [[Digital Equipment Corporation]]
| bits        = [[32-bit]]
| introduced  = 1988 (cancelled)
| image       = Dec-prism.svg
| version     = 
| design      = [[Reduced instruction set computer|RISC]]
| type        = 
| encoding    = 
| branching   = 
| endianness  = 
| page size   = 
| extensions  = 
| open        = 
| predecessor = 
| successor   = [[DEC Alpha]]
| registers   =
*64× 32-bit general purpose registers
*16× 64-bit vector registers
}}

'''PRISM''' ('''P'''arallel '''R'''educed '''I'''nstruction '''S'''et '''M'''achine)<ref>{{cite web |title=Sketch of DEC PRISM |author=Mark Smotherman 
|url=https://people.cs.clemson.edu/~mark/prism.html
|quote=PRISM (Parallel Reduced Instruction Set Machine) ... first draft of PRISM architecture in August 1985; DEC cancels the project in 1988 in favor of a MIPS-based ...}}</ref> 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 workstation]]s. 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 Technologies|MIPS]]-based systems.<ref>{{cite web|url=http://www.bitsavers.org/pdf/dec/prism/memos/880617_PRISM_killed.pdf|date=1988-06-17|title=PRISM killed|access-date=2021-01-05|website=bitsavers.org}}</ref> An operating system codenamed [[DEC MICA|MICA]] was developed for the PRISM architecture, which was intended as a replacement for both [[OpenVMS|VAX/VMS]] and [[ULTRIX]] on PRISM, although a standalone PRISM ULTRIX port was eventually created when the original multi-personality goal of MICA proved infeasible.<ref>{{cite web|url=http://www.bitsavers.org/pdf/dec/prism/memos/880530_Cutler_PRISM_vs_MIPS.pdf|title=DECwest/SDT Agenda|author=Dave Cutler|date=1988-05-30|website=bitsavers.org}}</ref><ref>{{cite web| url=https://www.computerhistory.org/collections/catalog/102781228| title=DECWEST engineering : today, tomorrow, and the future, part 2 of 2| website=computerhistory.org| publisher=DEC| date=1988-04-20| access-date=2025-05-21}}</ref>

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.