Hard Disk Storage - Upgrading and Repairing PCs - page 611

morequickly.Latencyfiguresformostpopulardriverotational speedsareshownin Table
9.13 .
Table 9.13 Hard Disk Rotation Speeds and Their Latencies
Many drives today spin at 7,200 rpm, resulting in a latency time of only 4.17ms, whereas
others spin at 10,000 rpm or even 15,000 rpm, resulting in incredible 3.00ms or 2.00ms
latency figures. In addition to increasing performance where real-world access to data is
concerned,spinningtheplattersmorequicklyalsoincreasesthedata-transferrateafterthe
heads arrive at the desired sectors.
Average Access Time
A measurement of a drive's average access time is the sum of its average seek time plus
latency. The average access time is usually expressed in milliseconds.
Ameasurementofadrive'saverageaccesstime(averageseektimepluslatency)provides
the average total amount of time required for the drive to access a randomly requested
sector.
Cache Programs and Caching Controllers
At the software level, disk cache programs such as SMARTDRV (DOS) and VCACHE
(Windows) can have a major effect on disk drive performance. These cache programs
hook into the BIOS hard drive interrupt and intercept the read and write calls to the disk
BIOS from application programs and device drivers.
When an application program wants to read data from a hard drive, the cache program in-
tercepts the read request, passes the read request to the hard drive controller in the usual
way, saves the data read from the disk in its cache memory buffer, and then passes the
databacktotheapplicationprogram.Dependingonthesizeofthecachebuffer,datafrom
numerous sectors can be read into and saved in the buffer.
When the application wants to read more data, the cache program again intercepts the re-
quest and examines its buffers to see whether the requested data is still in the cache. If so,
the program passes the data back from the cache to the application immediately, without