A control bus is what a computer’s central processing unit (CPU) uses to communicate with other devices within the machine through a set of physical connections, such as wires or printed circuit boards. It is a diverse collection of signals, including read, write, and interrupt, that allows the CPU to direct and monitor what different parts of the computer are doing. This is one of three bus types that make up the system or computer bus. Its exact composition varies between processors.
The expansion bus allows the computer’s motherboard to communicate with the hard drive.
In general, the purpose of any bus is to decrease the number of paths needed for communication between computer components. A bus allows communication between components on a data channel and is characterized by the amount of information it can transmit at once. The amount of data is expressed in bits and corresponds to the number of physical lines through which the information is sent. For example, a 32-wire flat cable can send 32 bits in parallel.
Most modern computers have a large number of buses that connect all sorts of different areas.
Each computer generally has an internal bus and an expansion bus. The internal or front bus facilitates communication between the CPU and main memory, while the expansion or input/output bus connects motherboard components such as hard drives and ports. Most system buses are usually made up of 50 to 100 separate physical lines for communication. These lines are subdivided into three subsets or types of buses: the address or memory bus, the data bus, and the command or control bus.
The control bus is bidirectional; transmits CPU command signals and hardware response signals. It helps the CPU to synchronize its command signals with slower computer components and external devices. As a result, the control bus consists of control lines, each of which sends a specific signal, such as read, write, and interrupt. The control lines that make up a control bus differ among processors, but most include system clock lines, status lines, and enable byte lines.
For example, a computer’s CPU will use the data bus to transmit information to and from main memory. The control bus allows the CPU to determine if and when the system is sending or receiving this data. This is because a control bus has a read and write control line that determines the direction of information flow (memory to CPU or CPU to memory). If the CPU needs to write any data to main memory, it will signal (turn on) the write control line of the control bus. Sending a signal on the read control line allows the CPU to receive data from memory.
The other types of buses that make up a system bus are data and address buses. The data bus moves instructions and information between all the functional components of the computer. It is bidirectional and can only transmit in one direction at a time. The data bus transmits information between the CPU and the memory and also between the memory and the input/output section.
The address bus is unidirectional and works like a memory map. When the computer system needs to access a certain memory location or input/output device, it declares the appropriate address on the address bus. This address is recognized by the appropriate circuitry which instructs the corresponding memory or device to read or send data on the data bus. Only the device or memory location that matches the address on the address bus will respond.
The central processing unit uses the control bus to communicate with other devices within a computer through physical connections, including printed circuit boards.
