Access time refers to how fast the disk or memory can locate and begin retrieving (accessing) a specific piece of information or transfer data to the CPU. A low access time indicates a fast hard drive. Access time is measured in milliseconds, or ms. when making comparisons, look for 'memory access time' (measured in nanoseconds), Memory access time refers to the time it takes to transfer a character from memory to or from the processor, and/or 'disk access time' (measured in milliseconds) refers to the time it takes to place the read / write heads over the requested data. Sometimes the speed of a disk drive is measured by the rate at which it transfers data - in this case, a higher' data transfer rate' indicates a faster drive. For a disk drive the access time includes both the head SEEK TIME and the LATENCY.
For a disk, total access time is the sum of seek and rotational times to reach a particular record. RAM may have an access time of 80 nanoseconds or less, while hard disk access time could be 18 milliseconds or less.
For a floppy drive, the average access time is 25 ms; the access times for CD-ROMs approximately twice as long as for floppy disks.
For a drum or fixed-head disk, average access time is a half-revolution and maximum access time is a full revolution, since both have heads that are fixed over the data areas. Average access times for drums are 5 to 10 ms.