Reverse directory for facilitating accesses involving a lower-level cache

A single-chip multiprocessor system, comprising:

• a semiconductor chip;
• a plurality of processors within the semiconductor chip;
• an L2 cache within the semiconductor chip;
• a plurality of L1 caches within the semiconductor chip and coupled to the plurality of processors;
• wherein each of the plurality of L1 caches is configured to perform memory accesses through the L2 cache;
• wherein the plurality of L1 caches are organized as write-through caches, so that updates to the plurality of L1 caches are immediately written through to the L2 cache;
• and
• a reverse directory coupled to the L2 cache;
• wherein the reverse directory includes a fixed entry corresponding to each entry in each of the plurality of L1 caches, wherein each fixed entry identifies an associated entry in the L2 cache.

A method that operates in a computer system that includes a plurality of higher-level caches that access data through a lower-level cache, the method comprising:

• receiving a request from a higher-level cache to retrieve a line from the lower-level cache;
• and
• if the line is present within the lower-level cache,
• sending the line to the higher-level cache so that the line can be stored in the higher-level cache, and
• storing information in a reverse directory located at the lower-level cache to indicate that the line is stored in the higher-level cache;
• wherein the reverse directory includes entries for lines in the plurality of higher-level caches, wherein each entry identifies an associated entry in the lower-level cache.