Efficient Macro-code Emulation In Hardwired Pipelined Processors

Traditionally microcoded computers have been the ideal machines for implementing scalable architectures. These machines easily implement application-specific functionality in microcode and they allow architecturally transparent variation of cost/performance by trading off application code, microcode, and hardware. In contrast, hardwired machines are intrinsically incapable of implementing scalability, because they only implement a single level of interpretation. Recent RISC designs have introduced architectural features which partly resolve the scalability issues. They implement architectural openendness to allow application-specific functionality to be added to the architecture (by means of coprocessors and special function units). Additionally they define functions which, depending on application, cost, and performance, can be implemented in hardware or, by means of emulation, in software. Although identical from an abstract point of view, scalability by means of microprogramming and by means of emulation on a hardwired machine is significantly different. This paper describes the emulation facility provided in SCARCE (SCalable ARChitecture Experiment), a streamlined architecture specifically designed for a wide range of embedded applications, requiring high performance. While architecturally transparent, this emulation facility operates with little overhead (8 cycles), adds three control registers, and is always interruptible. By increasing the hardware investment, the overhead could be decreased to 4 cycles per trap.