Chapter 6: Allison Hamann, Chris Barile

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=876437

Sector Cache

A. Organziation

1) Sectors
2) Subsectors
3) Validity Bit

B. Load Procedure

1) Sector miss
2) Subsector miss

C. Advantages

Sector cache is where the cache is divided into sectors. These sectors correspond to a logical sector on the main storage device. Sectors are not loaded into the cache all at once, but in smaller pieces known as subsectors, which are similar to the cache lines in direct mapped cache.

When a process requests data from a disk sector that is not in the cache, a cache sector is assigned to the sector on the main storage device where the requested data is stored. Then a portion of that sector, otherwise known as the subsector, is loaded into the cache. The subsector's validity bit is then set to reflect that it has been loaded from the main storage. When data from other subsectors are requested, the system loads those subsectors into the cache sector and sets their validity bits. The sector is not removed from the cache until it is needed for another program.

One reason for this approach is that programs are generally organized in contiguous blocks on disk. Another is that data is first looked up by sector, and then by subsector, which means that it can be found much quicker, and the hardware to do the simultaneous comparison of tags is less expensive.