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PRISM (Parallel Reduced Instruction Set Machine) was a
32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...
RISC In computer engineering, a reduced instruction set computer (RISC) is a computer designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a complex instruction set comp ...
instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
(ISA) developed by
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president un ...
(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 A workstation is a special computer designed for technical or scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating systems. The term ''workstat ...
s. The
arithmetic logic unit In computing, an arithmetic logic unit (ALU) is a combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on floating point num ...
(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 Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
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 Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washi ...
and led the development of
Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...
. The MIPS-based workstations were moderately successful among DEC's existing Ultrix users but had little success competing against companies like
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, t ...
. 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 Alpha (original name Alpha AXP) is a 64-bit reduced instruction set computer (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit VAX complex instruction set compute ...
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 In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
(ISA), which was implemented in complex
microcode In processor design, microcode (μcode) is a technique that interposes a layer of computer organization between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. Microcode is a la ...
. The VMS
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
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 In computer operating systems, memory paging is a memory management scheme by which a computer stores and retrieves data from secondary storage for use in main memory. In this scheme, the operating system retrieves data from secondary stor ...
. 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 In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...
single-chip
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
s with performance similar to early VAX machines yet able to fit into a desktop pizza box form factor. Companies like
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, t ...
introduced
Motorola 68000 series The Motorola 68000 series (also known as 680x0, m68000, m68k, or 68k) is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and ...
-based
Unix workstation A workstation is a special computer designed for technical or scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating systems. The term ''workstat ...
s 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 In computer engineering, a reduced instruction set computer (RISC) is a computer designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a complex instruction set comp ...
processor. Around the same time, in 1979,
Dave Patterson David Glenn Patterson (born July 25, 1956) is a former pitcher in Major League Baseball who made 36 relief appearances for the Los Angeles Dodgers in its 1979 season. Listed at 6' 0", 170 lb., Patterson batted and threw right handed. He was ...
was sent on a
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from
University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant un ...
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. By the mid-1980s practically every company with a processor design arm began exploring the RISC approach. In spite of any official disinterest, DEC was no exception. 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 Palo Alto (; Spanish for "tall stick") is a charter city in the northwestern corner of Santa Clara County, California, United States, in the San Francisco Bay Area, named after a coastal redwood tree known as El Palo Alto. The city was es ...
was a high-performance ECL based design that started in 1982, intended to run
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
. SAFE (''Streamlined Architecture for Fast Execution'') was a
64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A ...
design that started the same year, designed by
Alan Kotok Alan Kotok (November 9, 1941 – May 26, 2006) was an American computer scientist known for his work at Digital Equipment Corporation (Digital, or DEC) and at the World Wide Web Consortium (W3C). Steven Levy, in his book '' Hackers: Heroes of t ...
(of
Spacewar! ''Spacewar!'' is a Space combat game, space combat video game developed in 1962 by Steve Russell (computer scientist), Steve Russell in collaboration with Martin Graetz, Wayne Wiitanen, Robert Alan Saunders, Bob Saunders, Steve Piner, and others. ...
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, MA Hudson is a town in Middlesex County, Massachusetts, United States, with a total population of 20,092 as of the 2020 census. Before its incorporation as a town in 1866, Hudson was a neighborhood and unincorporated village of Marlborough, Mas ...
fab, intended to be used as a
co-processor A coprocessor is a computer processor used to supplement the functions of the primary processor (the CPU). Operations performed by the coprocessor may be floating-point arithmetic, graphics, signal processing, string processing, cryptography or ...
in VAX machine. The same year Dave Cutler started the CASCADE project at DECwest in Bellevue, Washington.


PRISM

Eventually, Cutler was asked to define a single RISC project in 1985, selecting Rich Witek as the chief architect. 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. Through this early period, there were constant changes in the design as debates within the company argued over whether it should be 32- or 64-bit, aimed at a commercial or technical workload, and so forth. These constant changes meant the final ISA specification was not complete until September 1986. At the time, the decision was made to produce two versions of the basic concept, DECwest worked on a "high-end" ECL implementation known as Crystal, while the Semiconductor Advanced Development team worked on microPRISM, a
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
version. This work was 98% done 1985–86 and was heavily supported by simulations by Pete Benoit on a large VAXcluster. Through this era there was still considerable scepticism on the part of DEC engineering as a whole about whether RISC was really faster, or simply faster on the trivial five-line programs being used to demonstrate its performance. Based on the Crystal design, in 1986 it was compared to the then-fastest machine in development, the VAX 8800. The conclusion was clear: for any given amount of investment, the RISC designs would outperform a VAX by 2-to-1. In the middle of 1987, the decision was made that both designs be 64-bit, although this lasted only a few weeks. In October 1987, Sun introduced the Sun-4. Powered by a 16 MHz
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system develope ...
, a commercial version of Patterson's RISC design, it ran four times as fast as their previous top-end Sun-3 using a 20 MHz Motorola 68020. With this release, DEC once again changed the target for PRISM, aiming it solely at the workstation space. This resulted in the microPRISM being respecified as a 32-bit system while the Crystal project was canceled. This introduced more delays, putting the project far behind schedule. By early 1988 the system was still not complete; the CPU design was nearly complete, but the FPU and MMU, both based on the contemporary Rigel chipset for the VAX, were still being designed. The team decided to stop work on those parts of the design and focus entirely on the CPU. Design was completed in March 1988 and
taped out ''Taped'' is a 2012 Dutch thriller film directed by Diederik van Rooijen. The film won the Best Feature Film award at the 2012 Stony Brook Film Festival. Susan Visser was also nominated for the Golden Calf for Best Actress for her role in the ...
by April.


Cancellation

Throughout the PRISM period, DEC was involved in a major debate over the future direction of the company. As newer RISC-based workstations were introduced, the performance benefit of the VAX was constantly eroded, and the price/performance ratio completely undermined. Different groups within the company debated how to best respond. Some advocated moving the VAX into the high-end, abandoning the low-end to the workstation vendors like Sun. This led to the VAX 9000 program, which was referred to internally as the "IBM killer". Others suggested moving into the workstation market using PRISM or a commodity processor. Still others suggested re-implementing the VAX on a RISC processor. Frustrated with the growing number of losses to cheaper faster competitive machines, independently, a small skunkworks group in Palo Alto, outside of Central Engineering, focused on workstations and UNIX/ Ultrix, entertained the idea of using an off-the-shelf RISC processor to build a new family of workstations. The group carried out due diligence, eventually choosing the
MIPS R2000 The R2000 is a 32-bit microprocessor chip set developed by MIPS Computer Systems that implemented the MIPS I instruction set architecture (ISA). Introduced in January 1986, it was the first commercial implementation of the MIPS architecture and the ...
. This group acquired a development machine and prototyped a port of Ultrix to the system. From the initial meetings with MIPS to a prototype machine took only 90 days. Full production of a DEC version could begin as early as January 1989, whereas it would be at least another year before a PRISM based machine would be ready. When the matter was raised at DEC headquarters the company was split on which approach was better. Bob Supnik was asked to consider the issue for an upcoming project review. He concluded that while the PRISM system appeared to be faster, the MIPS approach would be less expensive and much earlier to market. At the acrimonious review meeting by the company's Executive Committee in July 1988, the company decided to cancel Prism, and continue with the MIPS workstations and high-end VAX products. The workstation emerged as the DECstation 3100. By this time samples of the microPRISM had been returned and were found to be mostly working. They also proved capable of running at speeds of 50 to 80 MHz, compared to the R2000's 16 to 20. This would have offered a significant performance improvement over the MIPS systems.


Legacy

By the time of the July 1988 meeting, the company had swung almost entirely into the position that the RISC approach was a workstation play. But PRISM's performance was similar to that of the latest VAX machines and the RISC concept had considerable room for growth. As the meeting broke up,
Ken Olsen Kenneth Harry "Ken" Olsen (February 20, 1926 – February 6, 2011) was an American engineer who co-founded Digital Equipment Corporation (DEC) in 1957 with colleague Harlan Anderson and his brother Stan Olsen. Background Kenneth Harry Olsen w ...
asked Supnik to investigate ways that Digital could keep the performance of VMS systems competitive with RISC-based Unix systems. A group of engineers formed a team, variously referred to as the "RISCy VAX" or "Extended VAX" (EVAX) task force, to explore this issue. By late summer, the group had explored three concepts, a subset of the VAX ISA with a RISC-like core, a translated VAX that ran native VAX code and translated it on-the-fly to RISC code and stored in a cache, and the ultrapipelined VAX, a much higher-performance CISC implementation. All of these approaches had issues that meant they would not be competitive with a simple RISC machine. The group next considered systems that combined both an existing VAX single-chip solution as well as a RISC chip for performance needs. These studies suggested that the system would inevitably be hamstrung by the lower-performance part and would offer no compelling advantage. It was at this point that Nancy Kronenberg pointed out that people ran VMS, not VAX, and that VMS only had a few hardware dependencies based on its modelling of interrupts and memory paging. There appeared to be no compelling reason why VMS could not be ported to a RISC chip as long as these small bits of the model were preserved. Further work on this concept suggested this was a workable approach. Supnik took the resulting report to the Strategy Task Force in February 1989. Two questions were raised: could the resulting RISC design also be a performance leader in the Unix market, and should the machine be an open standard? And with that, the decision was made to adopt the PRISM architecture with the appropriate modifications, eventually becoming the
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whi ...
, and began the port of VMS to the new architecture. When PRISM and MICA were cancelled, Dave Cutler left Digital for
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washi ...
, where he was put in charge of the development of what became known as
Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...
. Cutler's architecture for NT was heavily inspired by many aspects of MICA.


Design

In terms of
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
operations, the PRISM architecture was similar to the MIPS designs. Of the 32-bits in the instructions, the 6 highest and 5 lowest bits were the instruction, leaving the other 21 bits of the word for encoding either a constant or
register Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), th ...
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 window In computer engineering, register windows are a feature which dedicates registers to a subroutine by dynamically aliasing a subset of internal registers to fixed, programmer-visible registers. Register windows are implemented to improve the perfo ...
s that were a hallmark of the other major RISC design, Berkeley RISC. The PRISM design was notable for several aspects of its
instruction set In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
. Notably, PRISM included Epicode (''extended processor instruction code''), which defined a number of "special" instructions intended to offer the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
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.


References


Bibliography

* * * * Prism documents a
bitsavers.org


Further reading

* Bhandarkar, Dileep P. (1995). ''Alpha Architecture and Implementations''. Digital Press. * Bhandarkar, D. et al. (1990
"High performance issue orientated architecture"
''Proceedings of Compcon Spring '90'', pp. 153–160. * Conrad, R. et al. (1989)
"A 50 MIPS (peak) 32/64 b microprocessor"
''ISSCC Digest of Technical Papers'', pp. 76–77. {{CPU technologies Instruction set architectures Information technology projects