A perspective on the 801/Reduced Instruction Set Computer

From the earliest days of computers until the early 1970s, the trend in computer architecture was toward increasing complexity. This complexity revealed itself through the introduction of new instructions that matched the application areas. Microcode was an im-plementation technique that greatly facilitated this trend; thus, most computers were implemented using microcode. In 1975, work began at the Thomas J. Watson Research Center on an experimental minicom-puter. This project, termed the 801 project, questioned the trend toward complexity in computer architecture. It was observed that most of the complex instructions were seldom used. Thus, a computer could be de-signed with only simple instructions without drastically increasing the path length or number of instructions required to implement an application. This made it pos-sible to implement a machine without resorting to mi-crocode, which improved performance. This paper de-scribed the background and evolution of these ideas in the context of the 801 experimental minicomputer proj-ect. omputers are unlike other tools in that they are C truly general-purpose instruments. The interface seen by an airline ticket agent is that of a machine that makes airline reservations. A secretary sees a text-editing machine. To the applications programmer, who uses a high-level language like FOR-TRAN or COBOL, the computer is a FORTRAN or COBOL machine. However, the assembly language programmer and compiler writer see the underlying machine architecture, an IBM System/370, a DEC VAX, or some other instruction set. A computer architecture is an abstract description of a machine; interest in computer architectures tran-scends any particular machine. Thus, there can be many models of System/370 or VAX, all of which can execute the same program. Because performanc