DEC PRISM
Encyclopedia
PRISM was a 32-bit
RISC instruction set architecture (ISA) developed by Digital Equipment Corporation
(DEC). It was the final outcome of a number of DEC research projects from the 1982–1985 time-frame, and was at the point of delivering silicon in 1988 when the management canceled the project. The next year work on the Alpha
started, based heavily on the PRISM design.
Titan from DEC's Western Research Laboratory (WRL) in Palo Alto, California
was a high-performance ECL
based design that started in 1982, intended to run Unix
. SAFE (Streamlined Architecture for Fast Execution) was a 64-bit
design that started the same year, designed by Alan Kotok
(of Spacewar! 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 fab, intended to be used as a co-processor in VAX
machine. The same year Dave Cutler
started the CASCADE project at DECwest in Bellevue, Washington.
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 bit
s 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 window
s 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.
s 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 workstations. Others suggested moving into the workstation market using a commodity processor. Still others suggested re-implementing the VAX on a RISC processor.
This led to considerable problems with turf war
s between the various groups. Competition between the divisions delayed the architecture review, which wasn't closed until 1986. Work on associated support chips, the memory management unit
and floating point unit
, were later interrupted by yet another debate on whether or not the design should be 32- or 64-bit. The MicroPRISM design was not finalized until April 1988.
Frustrated with the growing number of losses to cheaper faster competitive machines, independently, a small group 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 as the desired foundation. This group acquired a development machine and, in cooperation with hand-picked members of the Ultrix Engineering Group, prototyped a port of Ultrix to the MIPS R2000 system. From the initial meetings with MIPS to a prototype machine took only 90 days, with full production able to start by January 1989, resulting in the DECstation 3100
and family. At a meeting reviewing the various projects in July 1988, the company decided to cancel PRISM, and continue with the MIPS workstations and high-end VAX products.
Ironically, every attempt to produce a faster VAX that could compete with newer workstations was essentially a failure. The VAX 9000
ran into delays, and by the time it shipped newer Unix workstations had already surpassed it in performance, at a tiny fraction of the cost (or size). Apparently aware of this danger, at the very same meeting where PRISM was canceled, Ken Olsen
started a new project to continue exploring a RISC-based VAX. This indirectly led to the formation of the Alpha
project the next year.
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
RISC instruction set architecture (ISA) developed by Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
(DEC). It was the final outcome of a number of DEC research projects from the 1982–1985 time-frame, and was at the point of delivering silicon in 1988 when the management canceled the project. The next year work on the Alpha
DEC Alpha
Alpha, originally known as Alpha AXP, is a 64-bit reduced instruction set computer instruction set architecture developed by Digital Equipment Corporation , designed to replace the 32-bit VAX complex instruction set computer ISA and its implementations. Alpha was implemented in microprocessors...
started, based heavily on the PRISM design.
Background
In the early 1980s DEC was a huge success, flush with cash and infused with a feeling of invincibility. Projects were started all over the company to chase the "next big thing", with little or no overall direction or managerial oversight. RISC computing was one of those next big things, and in the period from 1982 to 1985 no less than four attempts were made to create a RISC chip at different divisions.Titan from DEC's Western Research Laboratory (WRL) in Palo Alto, California
Palo Alto, California
Palo Alto is a California charter city located in the northwest corner of Santa Clara County, in the San Francisco Bay Area of California, United States. The city shares its borders with East Palo Alto, Mountain View, Los Altos, Los Altos Hills, Stanford, Portola Valley, and Menlo Park. It is...
was a high-performance ECL
ECL
ECL may stand for:*ECL programming language, an extensible programming language developed at Harvard*ECL, data-centric programming language for Big Data, a declarative, data centric programming language used for data intensive supercomputing...
based design that started in 1982, intended to run Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
. SAFE (Streamlined Architecture for Fast Execution) was a 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
design that started the same year, designed by Alan Kotok
Alan Kotok
Alan Kotok was an American computer scientist known for his work at Digital Equipment Corporation and at the World Wide Web Consortium...
(of Spacewar! fame) and Dave Orbits and intended to run VMS
VMS
- Communication and transportation :* Voice Mail System, automated telephone messaging* Video Messaging Service , video messaging for 3G handsets* VMS MobiFone, one of the largest mobile phone operators in Vietnam...
. HR-32 (Hudson, RISC, 32-bit) started in 1984 by Rich Witek and Dan Dobberpuhl at the Hudson fab, intended to be used as a co-processor in VAX
VAX
VAX was an instruction set architecture developed by Digital Equipment Corporation in the mid-1970s. A 32-bit complex instruction set computer ISA, it was designed to extend or replace DEC's various Programmed Data Processor ISAs...
machine. The same year Dave Cutler
Dave Cutler
David Neil Cutler, Sr. is an American software engineer, designer and developer of several operating systems including RSX-11M, VMS and VAXELN at Digital Equipment Corporation and Windows at Microsoft.- Personal history :...
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. 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
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
operations, the PRISM architecture was similar to the MIPS
MIPS Technologies
MIPS Technologies, Inc. , formerly MIPS Computer Systems, Inc., is most widely known for developing the MIPS architecture and a series of pioneering RISC chips. MIPS provides processor architectures and cores for digital home, networking and mobile applications.MIPS Computer Systems Inc. was...
designs. Of the 32-bit instructions, the 6 highest and 5 lowest bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s were the instruction, leaving the rest of the word for encoding either a constant
Constant (programming)
In computer programming, a constant is an identifier whose associated value cannot typically be altered by the program during its execution...
or register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
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 window
Register window
In computer engineering, the use of register windows is a technique to improve the performance of a particularly common operation, the procedure call...
s that were a hallmark of the "other" design, Berkeley RISC
Berkeley RISC
Berkeley RISC was one of two seminal research projects into RISC-based microprocessor design taking place under ARPA's VLSI project. RISC was led by David Patterson at the University of California, Berkeley between 1980 and 1984, while the other was taking place only a short drive away at Stanford...
/SPARC
SPARC
SPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
.
The PRISM design was notable for several aspects of its instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
, however. Notably, PRISM included Epicode (extended processor instruction code), which defined a number of "special" instructions intended to offer the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
a stable ABI
Application binary interface
In computer software, an application binary interface describes the low-level interface between an application program and the operating system or another application.- Description :...
across multiple implementations. Epicode was given its own set of 22 32-bit registers to use. A set of vector processing
Vector processor
A vector processor, or array processor, is a central processing unit that implements an instruction set containing instructions that operate on one-dimensional arrays of data called vectors. This is in contrast to a scalar processor, whose instructions operate on single data items...
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
Emitter coupled logic
In electronics, emitter-coupled logic , is a logic family that achieves high speed by using an overdriven BJT differential amplifier with single-ended input, whose emitter current is limited to avoid the slow saturation region of transistor operation....
implementation known as Crystal, while the Semiconductor Advanced Development team worked on MicroPRISM, a CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
version. MicroPRISM was finished first and was sent for test fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...
in April 1988. Additionally, Cutler led development on a new microkernel
Microkernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
-based operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
code-named Mica, which was to offer Unix-like and VMS
VMS
- Communication and transportation :* Voice Mail System, automated telephone messaging* Video Messaging Service , video messaging for 3G handsets* VMS MobiFone, one of the largest mobile phone operators in Vietnam...
-like "personalities" on top of a common substrate of services.
Friction and cancellation
Throughout the PRISM period, DEC was involved in a major debate over the future direction of the company. As newer workstationWorkstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
s were introduced, the performance benefit of the VAX was constantly eroded, and the price/performance ratio
Price/performance ratio
In economics and engineering, the price/performance ratio refers to a product's ability to deliver performance, of any sort, for its price. Generally speaking, products with a higher price/performance ratio are more desirable, excluding other factors....
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 workstations. Others suggested moving into the workstation market using a commodity processor. Still others suggested re-implementing the VAX on a RISC processor.
This led to considerable problems with turf war
Turf war
According to Wordnet the definition of a turf war is "a bitter struggle for territory or power or control or rights". For example: a turf war erupted between street gangs; the president's resignation was the result of a turf war with the board of directors. In larger companies Turf wars could...
s between the various groups. Competition between the divisions delayed the architecture review, which wasn't closed until 1986. Work on associated support chips, the memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
and floating point unit
Floating point unit
A floating-point unit is a part of a computer system specially designed to carry out operations on floating point numbers. Typical operations are addition, subtraction, multiplication, division, and square root...
, were later interrupted by yet another debate on whether or not the design should be 32- or 64-bit. The MicroPRISM design was not finalized until April 1988.
Frustrated with the growing number of losses to cheaper faster competitive machines, independently, a small group 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 as the desired foundation. This group acquired a development machine and, in cooperation with hand-picked members of the Ultrix Engineering Group, prototyped a port of Ultrix to the MIPS R2000 system. From the initial meetings with MIPS to a prototype machine took only 90 days, with full production able to start by January 1989, resulting in the DECstation 3100
DECstation
The DECstation was a brand of computers used by DEC, and refers to three distinct lines of computer systems—the first released in 1978 as a word processing system, and the latter two both released in 1989. These comprised a range of computer workstations based on the MIPS architecture and a...
and family. At a meeting reviewing the various projects in July 1988, the company decided to cancel PRISM, and continue with the MIPS workstations and high-end VAX products.
Ironically, every attempt to produce a faster VAX that could compete with newer workstations was essentially a failure. The VAX 9000
VAX 9000
The VAX 9000, code named Aridus, was a family of high-end minicomputers developed and manufactured by Digital Equipment Corporation using processors implementing the VAX instruction set architecture . The VAX 9000 was positioned by Digital as its first mainframe...
ran into delays, and by the time it shipped newer Unix workstations had already surpassed it in performance, at a tiny fraction of the cost (or size). Apparently aware of this danger, at the very same meeting where PRISM was canceled, Ken Olsen
Ken Olsen
Kenneth Harry Olsen was an American engineer who co-founded Digital Equipment Corporation in 1957 with colleague Harlan Anderson.-Background:...
started a new project to continue exploring a RISC-based VAX. This indirectly led to the formation of the Alpha
DEC Alpha
Alpha, originally known as Alpha AXP, is a 64-bit reduced instruction set computer instruction set architecture developed by Digital Equipment Corporation , designed to replace the 32-bit VAX complex instruction set computer ISA and its implementations. Alpha was implemented in microprocessors...
project the next year.
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.