There are three basic methods by which data can be read from or written to a peripheral device and RAM (Random Access Memory). These input / output methods and communications of microcomputer are:
- Programmed I/O
- Interrupt I/O and
- Direct memory access
Programmed Input / Output (I/O)
In this technique, the microprocessor is in direct control of all transfers and other I/O operations. This is accomplished with specific input or output instructions.
When an input operation is desired, the microprocessor simply issues and input command and awaits the arrival of the data at the bus. From the bus the data are moved to memory.
Similarly, if an output operation is desired, the microprocessor transmits data to the bus and issues a command to the output device through the appropriate interface.
Once the data transfer is initiated, the microprocessor mist waits for its completion and the bus to be freed before beginning a new transfer. This method is commonly used personal computers because it is effective and easily implemented.
Interrupt Input / Output (I/O)
This is a device initiated transfer. An interrupt transfer involves the I/O device sending a request to the processor through interrupt input (INT, IRQ etc.) to inform the CPU that the I/O device is ready for data transfer.
In response, the CPU interrupts the execution of its current program and jumps to a new program called interrupt service routine which contains instructions to transfer data to or from the interrupting device.
Direct Memory Address
This method is fastest is the fastest of the methods. It requires a multi-bus architecture and allows the microprocessor to be bypassed completely. A special direct memory access controller is connected between RAM and an input or output device.
This method is the most expensive of all and therefore is not used except with very high-speed input/output devices and only when absolutely necessary.
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