Microsoft Word 2007.pdf

Input/Output Methods 

There are three basic methods by which data can be read (or input) from or written (or output) to a peripheral device and RAM. These methods are referred to as: programmed I/O, interrupt I/O, and direct memory access. In programmed I/O, the MPU directly controls all data transfers and other I/O operations. 

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This is accomplished with input or output instructions. When an input operation is desired, the MPU issues an input command and awaits the arrival of the data at the bus. From the bus the data are moved to memory. Similarly for an output operation, the MPU transmits the data to the bus and issues a command to the output device through the appropriate interface. Once data transfer is initiated, the MPU should wait for its completion and the bus to be freed before beginning a new transfer. This method is commonly used in personal computers. In the interrupt I/O method, the MPU does not wait for the input/output devices to complete their tasks. The control of the operation is given to a channel. The channel signals the MPU when the operation has been completed. This is accomplished by means of an interrupt. Upon completion of the execution of the current instruction, the MPU may then initiate another I/O operation. Direct memory address method is the fastest of the three methods. It requires a multibus architecture and allows the MPU to be bypassed completely. A direct memory access controller is connected between RAM and an input or output device. This method is the most expensive of the all and therefore is not used except with very high-speed input/output devices. 

Telecommunications Between Microcomputers 

Most microcomputers also support the transfer of data through telephone line. This is facilitated by means of an add-on device known as a modem. In telecommunications, one computer or terminal issues a command, or some form of output. This digital output from the computer, is modulated, or converted to an analog signal, by a modem interfaced to the sending computer. The signal is then carried over the telephone line and received at another modem some distance away. This second modem then demodulates, or converts the analog signal back to a computer-compatible digital signal. The name modem performs the functions of modulation and demodulation. Thus, with the aid of a modem and a serial interface to a microcomputer, communication can take place over long distances. The speed with which these communications take place is dependent on the modem employed.