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Definition: access time

(1) Memory access time is how long it takes for a character in memory to be transferred to or from the CPU. In a PC or Mac, fast RAM chips have an access time of 70 nanoseconds (ns) or less. SDRAM chips have a burst mode that obtains the second and subsequent characters in 10 ns or less.

(2) Disk access time is an average of the time between initiating a request and obtaining the first data character. It includes the command processing, the average seek time (moving the read/write head to the required track) and the average latency (rotation of disk to the required sector). This specification must be given as an average, because seek times and latency can vary depending on the current position of the head and platter.

Fast hard disks have access times of 10 milliseconds (ms) or less. This is a common speed measurement, but overall disk performance is significantly influenced by channel speed (transfer rate), interleaving and caching. See cache, seek time and latency.