Contents 1 Introduction
Changing trends in technologies, notably cheaper and faster memory hierarchies, have made it worthwhile to revisit many hardware-oriented design decisions made in previous decades. Hardware-oriented designs, in which one uses special-purpose hardware to perform some dedicated function, are a response to the high cost of executing instructions out of memory; when caches are expensive, slow, and/or in scarce supply, it is a perfectly reasonable reaction to build hardware state machines that do not compete with user applications for cache space and do not rely on the performance of the caches. In contrast, when the caches are large enough to withstand competition between the application and operating system, the cost of executing operating system functions out of the memory subsystem decreases significantly, and software-oriented designs become viable. Software-oriented designs, in which one dispenses with special-purpose hardware and instead performs the same function entirely in software, can offer increased flexibility over hardware state machines at a modest cost in performance. One current example is the translation of x86 instructions by Transmeta's code-morphing software layer [11]; this performs the same type of function as the front-end of the Pentium Pro/II/III pipeline, which turns x86 instructions into RISC-like uops in hardware [16].
