Mechanism to identify instruction word boundaries in cache

摘要

A mechanism which manages variable length instructions in cache is comprised of three cooperating elements designed to optimize self modifying code and anticipate next instructions for branch operand management. A content addressable memory (CAM) stores addresses of lines which have been accessed for instruction fetching. In a system having modifiable instruction stream (i.e., store to instruction stream), when the CAM matches, the system must retire certain instructions, flush instructions and then fetch the modified instruction stream. Boundary identification logic examines a field in each cache byte to determine the nature of the byte. This field is initially cleared at the time the cache line is loaded and filled with the line is fetched. An anticipation buffer designed to minimize the circuitry necessary for fetches across cache lines is loaded with sequentially anticipated prefetched instructions from the cache. These anticipated instructions can then be concatenated by a fetch aligner.