Functional view and operations of a computer

The functional view of a computer shows it as a system that performs operations on data under the control of a program. It has four main functions: data movement, data storage, data processing and control.

            +-----------------------+
            |   Control mechanism   |
            +-----------+-----------+
                        |  (controls all parts)
            +-----------+-----------+
 Outside    |   Data movement       |
 world <===>|   apparatus           |
            |     |           |     |
            |   Data        Data    |
            |   storage     processing
            +-----------------------+

The four functions

1. Data processing: the computer performs arithmetic and logic operations on data (add, compare, shift). The ALU does this work.
2. Data storage: the computer keeps data and programs. Short-term storage is in registers and main memory; long-term storage is on disks and tapes.
3. Data movement: data is moved between the computer and the outside world, or inside it.
   • Input-output: data moves to or from a directly attached device (peripheral).
   • Data communication: data moves over a longer distance to or from a remote device.
4. Control: the control unit manages the other functions. It fetches instructions from memory and issues timing and control signals so that the operations happen in the right order.

Four possible types of operation

• Data movement: data passes from one peripheral to another, for example from the keyboard to the screen.
• Storage: data from the outside world is stored, for example a file downloaded from the network into memory.
• Processing of stored data: data in storage is processed and the result is stored back, for example a = b + c with all values in memory.
• Processing between storage and I/O: stored data is processed and the result is sent to an I/O device, for example printing a report.