Architecture implications in the design of microprocessors

Abstract

This paper examines how architecture, the definition of the instruction set and other facilities that are available to the user, can influence the implementation of a very large scale integration (VLSI) microsystem. The instruction set affects the system implementation in a number of direct ways. The instruction formats determine the complexity of instruction decoding. The addressing modes available determine not only the hardware needed (multiported register files or three-operand adders), but also the complexity of the overall machine pipeline as greater variability is introduced in the time it takes to obtain an operand. Naturally, the actual operations specified by the instructions determine the hardware needed by the execution unit. In a less direct way, the architecture also determines the memory bandwidth required. A few key parameters are introduced that characterize the architecture and can be simply obtained from a typical workload. These parameters are used to analyze the memory bandwidth required and indicate whether the system is CPU- or memory-limited at a given design point. The implications of caches and virtual memories are also briefly considered.