The access time is the time that elapses to initiate, execute and acknowledge a command to access a storage medium. It applies equally to magnetic memories, optical storage media, semiconductor memories and flash memories and varies greatly depending on the type of storage medium addressed.
For main memories, the access time ranges from a few nanoseconds to 100 ns and more. For external memories, the access time varies depending on the organization of the memory access and the type of storage system, and can range from a few milliseconds to many seconds, as in the case of tape drives.
In the case of floppy dis ks and hard disks, the access time is the time required for the read/write head to move to a specific position on a data carrier and for the data to be brought under the read/write head via the rotational movement of the hard disk. It therefore depends on the initial position of the read/write head and the position of the storage area. The average access time(AAT) of a hard disk is calculated from the time required for half a rotation. At a rotation speed of 7,200 rpm, the runtime for half a rotation is 4.2 ms, and at 5,400 rpm it is 5.8 ms. The access time is slightly higher and is between 8 ms and 10 ms at 7,200 rpm.
Solid-state drives (SSDs) do not have mechanical access, but direct access to flash memory, so the access time is only about one hundredth of the access time of hard disks and is about 0.1 ms to 0.2 ms.
In the case of semiconductor memories, static SRAMs and dynamic DRAMs, the access times have been reduced considerably in recent years. For example, the access times of static RAMs are about 5 ns to 10 ns, while those of DRAMs are higher, ranging from 40 ns to 60 ns.
For CD drives, the average access time depends on the rotation speed. This can be between 1x, 2x, 4x up to 53x. At the slow rotation speeds, the access time is about 400 ms and then drops over 100 ms at 12x speed, to about 80 ms at 32x speed.
- For digital tape drives, the access time is the time it takes for the tape to position the data under the magnetic head. Since the magnetic tape has to run at high speed until it reaches the desired position, the access times are in the range of seconds and even minutes, depending on the tape run.