Reference Work Entry

Encyclopedia of Algorithms

pp 943-947



  • Norbert ZehAffiliated withFaculty of Computer Science, Dalhousie University Email author 
  • , Ulrich MeyerAffiliated withDepartment of Computer Science, Goethe University Fankfurt am Main


Disk access model (DAM) External-memory model

Years and Authors of Summarized Original Work

  • 1988; Aggarwal, Vitter


The input/output model (I/O model) [1] views the computer as consisting of a processor, internal memory (RAM), and external memory (disk). See Fig. 1. The internal memory is of limited size, large enough to hold M data items. The external memory is of conceptually unlimited size and is divided into blocks of B consecutive data items. All computation has to happen on data in internal memory. Data is brought into internal memory and written back to external memory using I/O operations (I/Os), which are performed explicitly by the algorithm. Each such operation reads or writes one block of data from or to external memory. The complexity of an algorithm in this model is the number of I/Os it performs.
I/O-Model, Fig. 1

The I/O model

The parallel disk model (PDM) [15] is an extension of the I/O model that allows the external memory to consist of D ≥ 1 parallel disks. See Fig. 2. In this model, a single I/O operation i ...

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