The microcomputer has three local buses. These buses of computer are:
- Address bus
- Data bus
- Control bus
The buses connect the CPU (microprocessor) to each of the memory and I/O devices. The CPU is involved in sending or receiving information to or from memory location, input or output device, and a secondary memory device (FDD or HDD).
When the CPU sends data to a device or memory, it is called WRITE operation and when the CPU receives data it is called READ operation. The functions of buses are described now.
The address bus is unidirectional, information flows over it only in one direction, from the CPU to the memory or I/O devices. The CPU generates addresses on the lines of the address bus. Each of the addresses corresponds to one memory location or one I/O device.
When the CPU wants to communicate with (READ or WRITE), certain memory location or I/O device, it places the appropriate address on its address output.
This address is then decoded to select the desired memory location or I/O device. This decoding process usually requires address decoder circuits.
The data bus is bidirectional and data can flow the CPU through it. The data bus can be either input or output depending on whether the CPU performs a READ or a WRITE operation.
During READ operation the data bus receives data that has been placed on the data bus by memory or I/O device selected by the address. During WRITE operation the data bus acts as the output and places data on the data bus which are sent to selected memory location or I/O device.
The control bus consists of a set of signals that are used to synchronize the activities of separate microcomputer elements. Some of these control signals are sent by the CPU to the other components to tell them the type of operation in progress.
The I/O devices can send control signals to the CPU. Read/Write, Rest, interrupt are examples of control signals used in a microcomputer.
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