Access time

Access time

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Access time is the time delay or latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...

 between a request to an electronic system, and the access being completed or the requested data returned.
  • In a telecommunication
    Telecommunication
    Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

    s system, access time is the delay between the start of an access attempt and successful access. Access time values are measured only on access attempts that result in successful access.
  • In a computer
    Computer
    A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

    , it is the time interval between the instant at which an instruction control unit initiates a call for data
    Data
    The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

     or a request to store
    Computer storage
    Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

     data, and the instant at which delivery of the data is completed or the storage is started.

  • In disk drives, disk access time is the time required for a computer to process data from the processor
    Central processing unit
    The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

     and then retrieve the required data from a storage device, such as a hard drive. For hard disk drives, disk access time is determined by a sum of the spin-up time, seek time, rotational latency, and transfer time.
    • Spin-up
      Spin-up
      Spin-up refers to the process of a traditional hard disk drive accelerating its platters from a stopped state to an operational speed. The period of time taken by the drive to perform this process is referred to as its spin-up time, the average of which is a S.M.A.R.T. attribute. The required...

       time - is the time required to accelerate the disk to operating speed. Most drives are left spinning to improve access time, but drives may be spun down to reduce energy use or noise
      Quiet PC
      A quiet PC is a personal computer that makes little noise. Common uses for quiet PCs include video editing, sound mixing, home servers, and home theater PCs. A typical quiet PC uses quiet cooling and storage devices and energy-efficient parts....

       as for example in laptop computers.
    • Seek time - is the time for the actuator arm to reach the desired disk track
      Track (disk drive)
      A disk drive track is a circular path on the surface of a disk or diskette on which information is magnetically recorded and from which recorded information is read....

      .
    • Rotational latency - the delay for the rotation of the disk to bring the required disk sector
      Disk sector
      In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user data. Traditional formatting of these storage media provides space for 512 bytes or 2048 bytes of user-accessible data per sector...

       under the read-write mechanism. It greatly depends on rotational speed of a disk, measured in revolutions per minute
      Revolutions per minute
      Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...

       (RPM).
  • Transfer time - time during which data is actually read or written to medium, with a certain throughput
    Throughput
    In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or pass through a certain network node...

    .
In disk drive art the term Average Access Time usually refers to average seek time.

See also

  • Memory latency
  • Mechanical latency
  • rotational latency
  • Seek time
  • Comparison of latency and bandwidth
  • Throughput
    Throughput
    In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or pass through a certain network node...

  • Lead time
    Lead time
    A lead time is the latency between the initiation and execution of a process. For example, the lead time between the placement of an order and delivery of a new car from a manufacturer may be anywhere from 2 weeks to 6 months...