Intel i860

The Intel i860 (also 80860) was a RISC microprocessor from Intel, first released in 1989. The i860 was (along with the i960) one of Intel's first attempts at an entirely new, high-end instruction set since the failed Intel i432 from the 1980s. It was released with considerable fanfare, and obscured the release of the Intel i960 which many considered to be a better design. The i860 never achieved commercial success and the project was terminated in the mid-1990s.

Andy Grove blamed the i860's failure in the marketplace on Intel being stretched too thin:

“ We now had two very powerful chips that we were introducing at just about the same time: the 486, largely based on CISC technology and compatible with all the PC software, and the i860, based on RISC technology, which was very fast but compatible with nothing. We didn't know what to do. So we introduced both, figuring we'd let the marketplace decide. ... our equivocation caused our customers to wonder what Intel really stood for, the 486 or i860? ”

— Andy Grove,



Technical features

The i860 combined a number of features that were unique at the time, most notably its VLIW (Very Long Instruction Word) architecture and powerful support for high-speed floating point operations. The design mounted a 32-bit ALU along with a 64-bit FPU that was itself built in three parts, an adder, a multiplier, and a graphics processor. The system had separate pipelines for the ALU, floating point adder and multiplier, and could hand off up to three operations per clock. (I.e., two instructions - one integer instruction and one floating point multiply-and-accumulate instruction per clock.)

All of the buses were 64-bits wide, or wider. The internal memory bus to the cache, for instance, was 128-bits wide. Both units had thirty-two 32-bit registers, but the FPU used its set as sixteen 64-bit registers. Instructions for the ALU were fetched two at a time to use the full external bus. Intel always referred to the design as the "i860 64-Bit Microprocessor".

The graphics unit was unique for the era. It was essentially a 64-bit integer unit using the FPU registers. It supported a number of commands for SIMD-like instructions in addition to basic 64-bit integer math. Experience with the i860 influenced the MMX functionality later added to Intel's Pentium processors.

One unusual feature of the i860 was that the pipelines into the functional units were program-accessible, requiring the compilers to carefully order instructions in the object code to keep the pipelines filled. In traditional architectures these duties were handled at runtime by a scheduler on the CPU itself, but the complexity of these systems limited their application in early RISC designs. The i860 was an attempt to avoid this entirely by moving this duty off-chip into the compiler. This allowed the i860 to devote more room to functional units, improving performance. As a result of its architecture, the i860 could run certain graphics and floating point algorithms with exceptionally high speed, but its performance in general-purpose applications suffered and it was difficult to program efficiently