Intel I860 - Technical Features

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 "Core" 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 at least 64 bits wide. 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. The IEEE and Intel referred to the design as the "i860 64-Bit Microprocessor".

Intel i860 instructions acted on data sizes from 8-bit through 128-bit.

The graphics unit was unique for the era. It was essentially a 64-bit integer unit using the FPU registers as eight 128-bit 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 order instructions carefully 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 (see below).