Architectural Tradeoffs in the Design of MIPS-X

摘要

The design of a RISC processor requires a careful analysis of the tradeoffs that can be made between hardware complexity and software. As new generations of processors are built to take advantage of more advanced technologies, new and different tradeoffs must be considered. We examine the design of a second generation VLSI RISC processor, MIPS-X. MIPS-X is a single-chip 32-bit VLSI processor that uses a simplified instruction set, pipelining and a software code reorganizer. In the quest for higher performance, MIPS-X uses a deeper pipeline, a much simpler instruction set and achieves the goal of single cycle execution using a 2-phase, 20 MHz clock. This necessitated the inclusion of an on-chip instruction cache and careful consideration of the control of the machine. Many tradeoffs were made during the design of MIPS-X and this paper examines several key areas. They are: the organization of the on-chip instruction cache, the coprocessor interface, branches and the resulting branch delay, and exception handling. For each issue we present the most promising alternatives considered for MIPS-X and the approach finally selected. Working parts have been received and this gives us a firm basis upon which to evaluate the success of our design.