Predecode unit adapted for variable byte-length instruction set

Details Predecode unit adapted for variable byte-length instruction set Predecode unit adapted for variable byte-length instruction set

1995-04-12

1998-10-06

Treat, William M.

395386, 395393, 39580023, G06F 930

Patent

active

058190597

ABSTRACT:
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 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 simultaneously to a plurality of decode units which form fixed issue positions within the superscalar microprocessor. By utilizing the predecode information from the predecode unit, the instruction alignment unit may be implemented with a relatively small number of cascaded levels of logic gates, thus accommodating very high frequencies of operation. Instruction alignment to decode units may further be accomplished with relatively few pipeline stages. Finally, since 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, a relatively large amount of predecode information may be conveyed with a relatively small number of predecode bits. This thereby allows a reduction in the size of the instruction cache without compromising performance.

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