MIPS CPU

Since 1985, many processors implementing some version of the MIPS architecture have been designed and used widely.

MIPS microprocessors

The first MIPS microprocessor, the '' R2000'', was announced in 1985. It added multiple-cycle multiply and divide instructions in a somewhat independent on-chip unit. New instructions were added to retrieve the results from this unit back to the processor register file; these result-retrieving instructions were interlocked.

The R2000 could be booted either ''big-endian'' or ''little-endian''. It had thirty-one 32-bit general purpose registers, but no status register (''condition code register'' (CCR), the designers considered it a potential bottleneck), a feature it shares with the AMD 29000, the DEC Alpha, and RISC-V. Unlike other registers, the program counter is not directly accessible.

The R2000 also had support for up to four co-processors, one of which was built into the main ''central processing unit'' (CPU) and handled exceptions, traps and memory management, while the other three were left for other uses. One of these could be filled by the optional ''R2010'' floating-point unit (FPU), which had thirty-two 32-bit registers that could be used as sixteen 64-bit registers for double-precision.

The ''R3000'' succeeded the R2000 in 1988, adding 32 KB (soon raised to 64 KB) caches for instructions and data, and support for shared-memory multiprocessing in the form of a cache coherence protocol. While there were flaws in the R3000s multiprocessing support, it was successfully used in several successful multiprocessor computers. The R3000 also included a built-in ''memory management unit'' (MMU), a common feature on CPUs of the era. The R3000, like the R2000, could be paired with a ''R3010'' FPU. The R3000 was the first successful MIPS design in the market, and eventually over one million were made. A faster version of the R3000 running up to 40 MHz, the ''R3000A'' delivered a performance of 32 million instructions per second (MIPS), or ''VAX Unit of Performance'' (VUPs). The MIPS R3000A-compatible '' R3051'' running at 33.8688 MHz was the processor used in the Sony PlayStation though it didn't have FPU or MMU. Third-party designs include Performance Semiconductor's ''R3400'' and IDT's ''R3500'', both of them were R3000As with an integrated R3010 FPU. Toshiba's ''R3900'' was a virtually first system on a chip (SoC) for the early handheld PCs that ran Windows CE. A radiation-hardened variant for outer space use, the Mongoose-V, is a R3000 with an integrated R3010 FPU.

The ''R4000'' series, released in 1991, extended MIPS to a full 64-bit word design, moved the FPU onto the main die to form a single-chip microprocessor, and had a then high clock rate of 100 MHz at introduction. However, to achieve the clock frequency, the caches were reduced to 8 KB each and they took three cycles to access. The high clock rates were achieved through the method of ''deep pipelining'' (called ''super-pipelining'' then). The improved ''R4400'' followed in 1993. It had larger 16 KB primary caches, largely bug-free 64-bit operation, and support for a larger L2 cache.

MIPS, now a division of Silicon Graphics (SGI) named MTI, designed the low-cost ''R4200'', the basis for the even cheaper ''R4300i''. A derivative of this microprocessor, the NEC VR4300, was used in the Nintendo 64 game console.

Quantum Effect Devices (QED), a separate company started by former MIPS employees, designed the ''R4600'' ''Orion'', the ''R4700'' ''Orion'', the ''R4650'' and the ''R5000''. Where the R4000 had pushed clock frequency and sacrificed cache capacity, the QED designs emphasized large caches which could be accessed in just two cycles and efficient use of silicon area. The R4600 and R4700 were used in low-cost versions of the SGI Indy workstation as well as the first MIPS-based Cisco routers, such as the 36x0 and 7x00-series routers. The R4650 was used in the original WebTV set-top boxes (now Microsoft TV). The R5000 FPU had more flexible single precision floating-point scheduling than the R4000, and as a result, R5000-based SGI Indys had much better graphics performance than similarly clocked R4400 Indys with the same graphics hardware. SGI gave the old graphics board a new name when it was combined with R5000, to emphasize the improvement. QED later designed the ''RM7000'' and '' RM9000'' family of devices for embedded system markets like computer networking and laser printers. QED was acquired by the semiconductor manufacturer PMC-Sierra in August 2000, the latter company continuing to invest in the MIPS architecture. The ''RM7000'' included an integrated 256 KB L2 cache and a controller for optional L3 cache. The ''RM9xx0'' were a family of SOC devices which included northbridge peripherals such as memory controller, PCI controller, Gigabit Ethernet controller and fast I/O such as a HyperTransport port.

The ''R8000'' (1994) was the first superscalar MIPS design, able to execute two integer or floating point and two memory instructions per cycle. The design was spread over six chips: an integer unit (with 16 KB instruction and 16 KB data caches), a floating-point unit, three fully-custom secondary cache tag RAMs (two for secondary cache accesses, one for bus snooping), and a cache controller ASIC. The design had two fully pipelined double precision multiply-add units, which could stream data from the 4 MB off-chip secondary cache. The R8000 powered SGI's POWER Challenge servers in the mid-1990s and later became available in the POWER Indigo2 workstation. Although its FPU performance fit scientific users quite well, its limited integer performance and high cost dampened appeal for most users. The R8000 was sold for only a year and remains fairly rare.

In 1995, the ''R10000'' was released. This processor was a single-chip design, ran at a higher clock frequency than the R8000, and had larger 32 KB primary instruction and data caches. It was also superscalar, but its major innovation was out-of-order execution. Even with one memory pipeline and simpler FPU, the vastly improved integer performance, lower price, and higher density made the R10000 preferable for most customers.

Some later designs have been based upon R10000 core. The ''R12000'' used a 0.25 micrometre process to shrink the chip and achieve higher clock rates. The revised '' R14000'' allowed higher clock rates with added support for ''double data rate synchronous dynamic random-access memory'' (DDR SDRAM) static random access memory (SRAM) in the off-chip cache. Later iterations are named '' R16000'' and ''R16000A'', and feature higher clock rates and smaller die manufacturing compared with before.

Other members of the MIPS family include the ''R6000'', an emitter-coupled logic (ECL) implementation produced by Bipolar Integrated Technology. The R6000 introduced the MIPS II architecture. Its translation lookaside buffer (TLB) and cache architecture are different from all other members of the MIPS family. The R6000 did not deliver the promised performance benefits, and although it saw some use in Control Data machines, it quickly disappeared from the mainstream market.

History

First hardware

In 1981, John L. Hennessy began the ''Microprocessor without Interlocked Pipeline Stages'' ( MIPS) project at Stanford University to investigate reduced instruction set computer (RISC) technology. The results of his research convinced him of the future commercial potential of the technology, and in 1984, he took a sabbatical to found MIPS Computer Systems. The company designed a new architecture that was also named MIPS, and introduced the first MIPS implementation, the '' R2000'', in 1985. The R2000 was improved, and the design was introduced as the ''R3000'' in 1988. These 32-bit CPUs formed the basis of their company through the 1980s, used primarily in Silicon Graphics' (SGI) series of workstations and later Digital Equipment Corporation DECstation workstations and servers. The SGI commercial designs deviated from Stanford MIPS by implementing most of the interlocks in hardware, supplying full multiply and divide instructions (among others). The designs were guided, in part, by software architect Earl Killian who designed the MIPS III 64-bit instruction-set extension, and led the work on the R4000 microarchitecture.

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 in 1992 to guarantee the design would not be lost. The new SGI subsidiary was named MIPS Technologies.

Licensable architecture

In the early 1990s, MIPS began to license their designs to third-party vendors. This proved fairly successful due to the simplicity of the core, which allowed it to have many uses that would have formerly used much less able complex instruction set computer (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 enjoy similar success by licensing their SPARC core but was not nearly as successful. By the late 1990s, MIPS was a powerhouse in the embedded processor field. According to MIPS Technologies Inc., there was an exponential growth, with 48-million MIPS-based CPU shipments and 49% of total RISC CPU market share in 1997. MIPS was so successful that SGI spun off MIPS Technologies in 1998. In 2000s fully half of MIPS's income came from licensing their designs, while much of the rest came from contract design work on cores for third parties.

In 1999, MIPS Technologies replaced the previous versions of the MIPS architecture with two architectures, the 32-bit ''MIPS32'' (based on MIPS II with some added features from MIPS III, MIPS IV, and MIPS V) and the 64-bit ''MIPS64'' (based on MIPS V) for licensing. Nippon Electric Corporation (NEC), Toshiba, and SiByte (later acquired by Broadcom) each obtained licenses for the MIPS64 as soon as it was announced. Philips, LSI Logic and Integrated Device Technology (IDT) have since joined them. Today, the MIPS cores are one of the most-used "heavyweight" cores in the market for computer-like devices: handheld PCs, set-top boxes, etc.

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 (see next section). The MIPS design team that designed the ''R4300i'' 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 uses. 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 processor devices using the MIPS architecture. Raza Microelectronics, Inc. bought the product line from failing SandCraft and later produced devices that contained eight cores for the telecommunication and networking markets. Cavium, originally a security processor vendor also produced devices with eight CPU cores, and later up to 32 cores, for the same markets. Both of these firms designed their cores in-house, only licensing the architecture instead of buying cores from MIPS.

The desktop

Among the manufacturers which have made computer workstation systems using MIPS processors are SGI, MIPS Computer Systems, Inc., Whitechapel Workstations, Olivetti, Siemens-Nixdorf, Acer, Digital Equipment Corporation, NEC, and DeskStation.

Operating systems ported to the architecture include SGI's IRIX, Microsoft's Windows NT (through v4.0), Windows CE, Linux, FreeBSD, NetBSD, OpenBSD, UNIX System V, SINIX