Bus (computing)

A computer bus is a path for information.

It lets parts inside the machine talk. It can be made of wires.

Some buses connect parts inside the computer. These are called system buses. Other buses connect extra parts. These are called expansion buses. A USB is a type of bus. It connects things to the outside. The bus helps move data fast. It works like a tiny road for data. It is a very busy path!

A computer bus is a way to move data.

It acts like a shared path for parts to talk. This path can be made of wires or tiny traces.

These paths use a protocol. A protocol is a set of rules for talking. These rules help parts take turns. This stops them from crashing into each other.

There are different types of buses. System buses connect the CPU and memory. Expansion buses connect extra parts. A USB is a bus for outside devices. Some buses are parallel. This means they carry many bits at once on many wires. Other buses are serial. They send bits one by one. Serial buses can often be faster. They do not have the same timing problems as parallel ones.

Buses also have a width. This is the number of bits sent at once. An address bus tells the computer where to find data. It uses a physical address to point to a spot in memory. A 32-bit address bus can find over 4 billion spots. This helps the computer work very fast.

A computer bus is a communication system used to move data.

It works like a shared pathway between different parts of a computer. This pathway can be made of physical wires or tiny traces on a circuit board. It also includes software called a communication protocol. A protocol is a set of rules that tells devices how to talk. These rules make sure that only one device sends data at a time. This helps prevent conflicts so that information flows in an orderly way.

Buses can work in two main ways: parallel or serial. In a parallel bus, many bits of data travel at the same time on many different wires. However, as speeds increase, parallel buses face hard jobs like timing skew and interference. A serial bus sends data one bit at a time in a single stream. Even though they use fewer wires, serial buses can often be faster. This is because they do not have the same timing problems as parallel ones. Common examples of serial buses include USB, FireWire, and Serial ATA.

There are also different roles for these pathways. A system bus, or memory bus, connects the CPU to the computer's memory. This helps the brain of the computer reach its stored information quickly. An expansion bus connects extra parts, known as peripherals, to the system. For example, PCI Express is used for high-speed internal connections. USB is a very common bus for connecting devices on the outside. Some buses even provide power to the hardware they connect. This is a key difference between a bus and a network like Ethernet.

An address bus is a special type of bus used to find specific locations. When a device needs to read or write data, it sends a physical address on this bus. The width of the bus tells us how many memory locations it can find. For instance, a 32-bit address bus can reach 4,294,967,296 different spots. To save money on wires, some systems use a trick called address multiplexing. This means the address is sent in two equal parts during different cycles. This allows a 32-bit address to work using only 16 physical lines.

Computer buses have changed a lot over time. Early computers used bundles of wires that were called the digit trunk. These early systems often used interrupts to let parts talk to the CPU. An interrupt is a way for a device to say, "I need attention now." In 1958, IBM introduced channel controllers on the IBM 709 computer. These were like small, dedicated computers that handled data tasks on their own. This helped the main CPU focus on other jobs and made the whole system work better.

In computer architecture, a bus is a communication system designed to transfer data between components.

It acts as a shared physical pathway for information. This pathway consists of hardware like wires or optical fibers. It also includes software known as communication protocols. These protocols are sets of rules that manage data exchange. They ensure that multiple devices do not conflict when transmitting. By following these rules, the system maintains an orderly flow of information.

Buses are categorized by their specific roles within a system. A system bus, also called an internal or memory bus, connects the CPU to the memory. Expansion buses, or peripheral buses, allow the system to connect additional devices. For example, PCI Express (PCIe) provides high-speed internal connections. Meanwhile, the Universal Serial Bus (USB) connects external devices. Some buses also provide power to the connected hardware. This power-delivery feature distinguishes a bus from a network like Ethernet.

One critical component is the address bus. This bus specifies a physical address when a processor needs to read or write memory. The data itself is sent on a separate data bus. The width of the address bus determines the total addressable memory space. For instance, a 32-bit address bus can reach 4,294,967,296 unique memory locations. If each location holds one byte, the space is about 4 gigabytes. This width is a vital measurement of a system's capacity.

To reduce costs, engineers often use address multiplexing. Early processors used one dedicated wire for every single bit of address width. A 16-bit bus required 16 physical wires. As buses grew wider, this became too expensive in terms of chip pins. Multiplexing solves this by sending the address in two equal parts. In a 32-bit system, 16 lines send the first half of the address. This is immediately followed by the second half in a different cycle. Control signals called row-address strobe (RAS) and column-address strobe (CAS) manage this timing.

Buses can also be classified by how they transmit data. Parallel buses carry data words simultaneously across multiple wires. However, high speeds cause problems like timing skew and electromagnetic interference. Timing skew occurs when bits do not arrive at the same time. Serial buses send data one bit at a time in a stream. Serial buses often achieve higher data rates than parallel ones. They avoid crosstalk and timing skew because they use fewer electrical connections. Examples include USB, FireWire, and Serial ATA.

Modern computing has seen a major shift toward serial communication. This transition was supported by Moore's law. This law allowed for serializer/deserializer circuits to be built into integrated circuits. These circuits convert parallel data into serial streams and back again. The speed of a bus is measured in Hertz (Hz). It determines how many clock cycles occur per second. A Single Data Rate (SDR) bus performs one transfer per cycle. A Double Data Rate (DDR) bus performs two transfers per cycle.

Data transfer speed is also known as bandwidth. You can calculate the data rate by multiplying bits per cycle, frequency, and transfers per cycle. Some buses like PCIe use advanced encoding called PAM4. This method groups two bits into symbols to increase speed without raising frequency. Other systems use encoding like 128/130b for error correction. This can make the effective transfer rate slightly lower than the raw speed. These complex methods ensure that data remains accurate during high-speed travel.

Historically, the development of buses has been a collaborative effort. Groups like the IEEE Bus Architecture Standards Committee helped create standards. Early computer buses were simple bundles of wire called digit trunks. These were often separate for memory and peripherals. To improve efficiency, engineers developed interrupts. An interrupt allows a peripheral to signal the CPU for attention. In 1958, IBM introduced channel controllers on the IBM 709. These acted as small, dedicated computers to handle bus operations. This reduced the load on the main CPU and improved performance.