Online Computer Terms Dictionary - S

sector interleave

(Or sector map) The mapping from logical to physical sector numbers on a magnetic disk designed to optimise sequential reads and writes. Data is usually transferred to and from the disk in blocks or sectors where one sector lies within a continuous range of rotational angle of the disk. If logical sectors are assigned sequentially to physical sectors (0,1,2,...) then by the time one sector has been read and processed (e.g. writen to main memory) the start of the next logical sector will have passed the read/write head and will not be accessible until the disk's rotation brings it back under the head.

Staggering the physical sectors (e.g. 0,3,6,1,4,7,2,5,8) aims to allow just enough time deal with one sector before the next is accessible. This obviously depends on the relative speed of the rotation of the disk, sector size, sectors per track and the speed of transfer of sectors to main memory.

sector interleaving

sector interleave

sector map

sector interleave

sector mapping

In this scheme the memory and cache are divided into blocks of 2^m bytes (the cache line size). A sector consists of 2^n consecutive blocks.

When a block is cached, it is read into the correct position in any sector of the cache, given by discarding the bottom m address bits and taking the next n as the block number within the sector.

That whole sector is then tagged with the remaining upper address bits and the other blocks in the sector are marked as invalid. This scheme takes advantage of locality of reference to consecutive blocks and needs fewer tags thus reducing the cost of associative access to the tags.