Sep 022011

Early computer buses used parallel electric wires to transfer data. For example, an 8-bit bus used eight wires plus ground. Modern computer buses can use parallel or serial connections but they essentially serve the same purpose of transporting binary data encoded as electrical signals.

Computer buses are frequently divided into three categories:

  • Address bus
  • Data bus
  • Control bus

The address bus is used to specify a physical memory address. This can include primary memory (e.g. RAM and ROM) secondary memory (e.g. hard disk drives) and any other connected devices. Monitors, printers, scanners, and keyboards will all be mapped to memory.

The width of the address bus determines the amount of memory that can be addressed. A 32-bit bus can address 4,294,967,296 (2 ^ 32) bytes of memory. This explains the 4 GB memory restriction on 32-bit operating systems.

The data bus connects the microprocessor (CPU) with other devices mapped onto the system. This is often described pictorially using a memory map.

Unlike the address bus, the data bus is two-way (it supports read and write operations). The data bus is not limited to transferring data; it is also used to transfer program instructions.

Finally, the control bus carries commands from and returns status signals to the microprocessor. The control unit inside a CPU manages the internal control bus — internal to the CPU — and the external control bus.

  15 Responses to “Address bus, data bus, and control bus explained”

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  9. Explain why the data bus connect to various unit

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