Data need to be transferred between the CPU and primary memory as well as between the primary and secondary memory.
Data are transferred between different components of a computer system using physical wires called bus. For example, bus is used for data transfer between a USB port and hard disk or between a hard disk and main memory. Bus is of three types—
(i) Data bus to transfer data between different components,
(ii) Address bus to transfer addresses between CPU and main memory. The address of the memory location that the CPU wants to read or write from is specified in the address bus,
and (iii) Control bus to communicate control signals between different components of a computer. All these three buses collectively make the system bus, as shown in Figure. As the CPU interacts directly with main memory, any data entered from input device or the data to be accessed from hard disk needs to be placed in the main memory for further processing. The data is then transferred between CPU and main memory using bus.
The CPU places on the address bus, the address of the main memory location from which it wants to read data or to write data. While executing the instructions, the CPU specifies the read or write control signal through the control bus.
As the CPU may require to read data from main memory or write data to main memory, a data bus is bidirectional. But the control bus and address bus are unidirectional. To write data into memory, the CPU places the data on the data bus, which is then written to the specific address provided through the address bus. In case of read operation, the CPU specifies the address, and the data is placed on the data bus by a dedicated hardware, called memory controller. The memory controller manages the flow of data into and out of the computer's main memory.
