Adaptive interpretation as a means of exploiting complex instruction sets

Abstract

In this paper we concentrate on the effect of instruction set architecture on the performance potential of a computer system. These issues are key in considerations of what instruction set is most appropriate for the support of high level languages on general purpose machines. Two possible approaches are so called complex instruction sets, such as those of the VAX and IAPX 432, and the “reduced” instruction set of the RISC I [2] microcomputer, which is expected to have performance similar to that of a VAX 11-780. We propose a method of instruction set interpretation that takes advantage of the architectural features of complex instruction sets. These methods have been simulated executing real programs and in the case of the VAX instruction set have resulted in a typical improvement of a factor of two, assuming the same cycle time as the VAX 11-780. The techniques presented exploit the context available in a complex instruction and retain this information for use in subsequent execution of that instruction. Since this context is available only within a single instruction, low level instruction sets cannot benefit from this technique. In this sense the reduced instruction set machines as implemented in RISC are at their architectural limits, while complex instruction sets such as the VAX are far from theirs. For these reasons, complex instruction sets can have a significantly greater performance potential than a RISC instruction set for a given technology.