Gecko's CPU Library

MIPS architecture

Introduction: 1985

MIPS (originally an acronym for Microprocessor without Interlocked Pipeline Stages) is a RISC microprocessor architecture developed by MIPS Technologies. By the late 1990s it was estimated that one in three RISC chips produced were MIPS-based designs.

In 1981, a team led by John L. Hennessy at Stanford University started work on what would become the first MIPS processor. The basic concept was to dramatically increase performance through the use of deep instruction pipelines, a technique that was well known, but difficult to implement. Generally in a pipeline architecture, successive instructions in a program sequence will overlap in execution. Modules inside the CPU work in parallel so that the CPU will fetch and start executing an instruction before the preceding instruction is complete. In contrast, traditional designs of the era waited to complete an entire instruction before moving on, thereby leaving large areas of the CPU idle as the process continued.

One major barrier to pipelining was that it required interlocks to be set up to ensure that instructions that took multiple clock cycles to complete would stop the pipeline from loading more data — basically to pause while it completed. These interlocks can take a long time to set up, and were thought to be a major barrier to future speed improvements. A major aspect of the MIPS design was to fit every sub-phase (including memory access) of all instructions into one cycle, thereby removing any needs for interlocking, and permitting a single cycle throughput.

Although this design eliminated a number of useful instructions, notably things like multiply and divide which would take multiple (execution) steps, it was felt that the overall performance of the system would be dramatically improved because the chips could run at much higher clock rates. This ramping of the speed would be difficult with interlocking involved, as the time needed to set up locks is as much a function of die size as clock rate: adding the hardware needed might actually slow down the overall speed.

The elimination of these instructions became a contentious point. Many observers claimed the design (and RISC in general) would never live up to its hype. If one simply replaces the complex multiply instruction with many simpler additions, where is the speed increase? This overly-simple analysis ignored the fact that the speed of the design was in the pipelines, not the instructions.

In 1984 Hennessy was convinced of the future commercial potential of the design, and left Stanford to form MIPS Computer Systems. They released their first design, the R2000, in 1985, improving the design as the R3000 in 1988. These 32-bit CPUs formed the basis of their company through the 1980s, used primarily in SGI's series of workstations. These commercial designs deviated from the Stanford academic research by implementing most of the interlocks in hardware, supplying full multiply and divide instructions (among others).

In 1991 MIPS released the first 64-bit microprocessor, the R4000. However, MIPS had financial difficulties while bringing it to market. The design was so important to SGI, at the time one of MIPS' few major customers, that SGI bought the company outright in 1992 in order to guarantee the design would not be lost. As a subsidiary of SGI, the company became known as MIPS Technologies.

In the early 1990s MIPS started licensing their designs to third-party vendors. This proved fairly successful due to the simplicity of the core, which allowed it to be used in a number of applications that would have formerly used much less capable CISC designs of similar gate count and price - the two are strongly related; the price of a CPU is generally related to the number of gates and the number of external pins. Sun Microsystems attempted to follow their success by licensing their SPARC core, but it has never been anywhere near as successful. By the late 1990s MIPS was a powerhouse in the embedded processor field, and in 1997 the 48-millionth MIPS-based CPU shipped, making it the first RISC CPU to outship the famous 68k family. MIPS was so successful that SGI spun-off MIPS Technologies in 1998. Fully half of MIPS' income today comes from licensing their designs, while much of the rest comes from contract design work on cores that will then be produced by third parties.

In 1999 MIPS formalized their licensing system around two basic designs, the 32-bit MIPS32 (based on MIPS II with some additional features from MIPS III, MIPS IV, and MIPS V) and the 64-bit MIPS64 (based on MIPS V). NEC, Toshiba and SiByte (later acquired by Broadcom) each obtained licenses for the MIPS64 as soon as it was announced. Philips, LSI Logic and IDT have since joined them. Success followed success, and today the MIPS cores are one of the most-used "heavyweight" cores in the marketplace for computer-like devices (hand-held computers, set-top boxes, etc.), with other designers fighting it out for other niches. Some indication of their success is the fact that Freescale (spun-off by Motorola) uses MIPS cores in their set-top box designs, instead of their own PowerPC-based cores.

Since the MIPS architecture is licensable, it has attracted several processor start-up companies over the years. One of the first start-ups to design MIPS processors was Quantum Effect Devices (QED). The MIPS design team that designed the R4300 started the company SandCraft, which designed the R5432 for NEC and later produced the SR71000, one of the first out-of-order execution processors for the embedded market. The original DEC StrongARM team eventually split into two MIPS-based start-ups: SiByte which produced the SB-1250, one of the first high-performance MIPS-based systems-on-a-chip (SOC); while Alchemy Semiconductor (later acquired by AMD) produced the Au-1000 SoC for low-power applications. Lexra used a MIPS-like architecture and added DSP extensions for the audio chip market and multithreading support for the networking market. Due to Lexra not licensing the architecture, two lawsuits were started between the two companies. The first was quickly resolved when Lexra promised not to advertise their processors as MIPS-compatible. The second (about MIPS patent 4814976 for handling unaligned memory access) was protracted, hurt both companies' business, and culminated in MIPS Technologies giving Lexra a free license and a large cash payment.

Two companies have emerged that specialize in building Multi-core devices using the MIPS architecture. Raza Microelectronics Inc. purchased the product line from failing Sandcraft and later produced devices that contained 8 CPU cores that were targeted at the telecom and networking markets. Cavium Networks, originally a security processor vendor also produced devices with 8 CPU cores for the same markets. Both of these companies designed their cores in-house, just licensing the architecture instead of purchasing cores from MIPS.

MIPS designs are currently primarily used in many embedded systems such as the Series2 TiVo, Windows CE devices, Cisco routers, Foneras, and video game consoles like the Nintendo 64 and Sony PlayStation, PlayStation 2, and PlayStation Portable handheld system. Until late 2006 they were also used in many of SGI's computer products.

Source: Wikipedia, the free encyclopedia.