 Notes > Computer Systems > Device Control and Devices
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The following devices are associated with a typical computer workstation:
- 3 1/2 inch disk drive
- CD drive
- Fixed / hard disk
- Microphone
- Speakers
- Video camera
- Keyboard
- Mouse
- Screen or Visual Display Unit (VDU)
Devices that connect to a computer are controlled and managed by device Input/Output (I/O) controllers. These controllers translate and enable communication between peripherals and the internal workings of the computer. They tend to have their own small CPU and built-in memory in order to control its activities. The I/O controllers use the address and data buses that are connected to main memory as well as the computer's CPU. This enables the controller to interact with and monitor the signals being sent across the buses.
The CPU can therefore communicate with controllers attatched to the bus in a similar way that it communicates with main memory. In main memory there are actually sets of addresses assigned to controllers. These sets of addresses are known as "ports". When instructions are executed in the CPU, the memory locations that are referred to in the instruction can therefore be pointing to controllers and this means the instruction will apply to the controller and its attatched device. This system of having assigned memory address is known as "Memory Mapped I/O".
Direct Memory Access (DMA) involves a controller accessing main memory directly without the use of the CPU. This can significantly improve the performance of a computer as the CPU is free to carry out other tasks while the device controller carries out its operations on memory. The device is told initially where to put the data or where to retrieve it from in memory then the device becomes what is known as the "bus master" as it determines what data is sent over the bus to and from main memory.
Peripheral devices connected to device controllers do not only receive data and instructions from the computer (controller) but they send back signals to the controller also. This is in order to co-ordinate the activities of the device. This process of exchanging information is known as "handshaking". In the example of a printer, data is obviously sent to the printer to be printed out, but signals are sent back to the printer controller to indicate its status i.e. whether it is ready for more data, or it has finished printing, or if an error has occured etc...
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