This thesis introduces Programmable Reduced Instruction Set Computers (PRISC) as a new class of general-purpose computers. PRISC use RISC techniques as a base, but in addition to the conventional RISC instruction resources, PRISC offer hardware programmable resources which can be configured based on the needs of a particular application. This thesis presents the architecture, operating system, and programming language compilation techniques which are needed to successfully build PRISC. Performance results are provided for the simplest form of PRISC -- a RISC microprocessor with a set of programmable functional units consisting of only combinational functions. Results for the SPECint92 benchmark suite indicate that an augmented compiler can provide a performance improvement of 22% over the underlying RISC computer with a hardware area investment less than that needed for a 2 kilobyte SRAM. In addition, active manipulation of the source code leads to significantly higher local performance gains (250%-500%) for general abstract data types such as short-set vectors, hash tables, and finite state machines. Results on end-user applications that utilize these data types indicate a performance gain from 32%-213%.