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, but by the mid-1980s the consensus at DEC was that RISC was at least twice as efficient as CISC. The company had two options: Improve VAX as much as possible until the limitation of its 32-bit address space forced a replacement around 1999, or develop RISC as fast as possible. By the mid-1980s practically every company with a processor design arm began exploring the RISC approach. Despite 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 was a high-performance
ECL based design that started in April 1982, intended to run
Unix, and achieving this objective in December 1985. Titan ran a modified version of Ultrix called Tunix (Titan Unix) with rewritten process and memory management functions. The continuing performance gulf between the servers at WRL, which included these RISC-based Titan servers, and the VAX-based workstations being used as clients would eventually precipitate the development of the MIPS-based DECstation. Titan was accompanied by the MultiTitan project, running from mid-1984 until January 1988 which explored instruction-level parallelism, shared memory multiprocessing, explicit cache management, high-performance floating-point arithmetic, unified scalar and vector processing, and the use of a "software definition" for the architecture. In April 1988, this effort delivered samples of a microprocessor fabricated using a CMOS process that were tested within the existing Titan architecture, able to operate at frequencies up to between 70 MHz and 90 MHz depending on the individual part, but falling short of the desired 100 MHz. Without the accompanying floating-point and cache management chips, benchmarking could not be performed, with only estimates of 20 MIPS performance being made.
SAFE (
Streamlined Architecture for Fast Execution) was a
64-bit design that started in 1982, 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, MA 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.
PRISM DEC used the large profits from VAX to fund the multiple competing projects, which caused delays and uncertainty. 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 version. This work was 98% done by 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, 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 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. 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. Performance predictions based on these observations suggested a significant performance improvement over existing and announced RISC products from other vendors. However, without the accompanying floating-point unit, whose design had been halted, or the cache interface chip required for operating at such frequencies, which had been part of a cancelled project, floating-point performance predictions remained hypothetical.
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 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. When PRISM and MICA were cancelled, Dave Cutler left Digital for
Microsoft, where he was put in charge of the development of what became known as
Windows NT. Cutler's architecture for NT was heavily inspired by many aspects of MICA. ==Design==