Variable byte-length instructions using state of function bit of second byte of plurality of instructions bytes as indicative of whether first byte is a prefix byte

摘要

A superscalar microprocesor is provided that includes a predecode unit adapted for predecoding variable byte-length instructions. The predecode unit predecodes the instructions prior to their storage within an instruction cache. In one system, a predecode unit is configured to generate a plurality of predecode bits including a start bit, an end bit, and a functional bit for each instruction byte. The plurality of predecode bits associated with each instruction byte are collectively referred to as a predecode tag. An instruction alignment unit then uses the predecode tags to dispatch the variable byte-length instructions to a plurality of decode units within the superscalar microprocessor. The predecode unit is configured such that the meaning of the functional bit of a particular predecode tag is dependent upon the status of the start bit. The predecode unit is further configured to generate a functional bit associated with each byte of an instruction other than the starting byte, which indicate whether the associated byte is a prefix or opcode. The encoding of the predecode tags is such that a relatively large amount of predecode information may be conveyed with a relatively small number of predecode bits.