Interconnecting The Units Of A Computer

CPU sends data, instructions and information to the components inside the computer as well as to the peripherals and devices attached to it. Bus is a set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer. The different components of computer, i.e., CPU, I/O unit, and memory unit are connected with each other by a bus. The data, instructions and the signals are carried between the different components via a bus. The features and functionality of a bus are as follows

· A bus is a set of wires used for interconnection, where each wire can carry one bit of data.

· A bus width is defined by the number of wires in the bus.

· A computer bus can be divided into two types—Internal Bus and External Bus.

· The Internal Bus connects components inside the motherboard like, CPU and system memory. It is also called the System Bus. Figure shows interaction between processor and memory.

Interaction between CPU and memory

The External Bus connects the different external devices, peripherals, expansion slots, I/O ports and drive connections to the rest of computer. The external bus allows various devices to be attached to the computer. It allows for the expansion of computer’s capabilities. It is generally slower than the system bus. It is also referred to as the Expansion Bus.

· A system bus or expansion bus comprise of three kinds of buses — data bus, address bus and control bus.

· The interaction of CPU with memory and I/O devices involves all the three buses.

a) The command to access the memory or the I/O device is carried by the control bus. o

b) The address of I/O device or memory is carried by the address bus.

c) The data to be transferred is carried by the data bus.

Figure shows interaction between processor, memory and the peripheral devices.

System Bus

Data Bus transfers data between the CPU and memory. The bus width of a data bus affects the speed of computer. The size of data bus defines the size of the processor. A processor can be 8, 16, 32 or 64-bit processor. An 8–bit processor has 8 wire data bus to carry 1 byte of data. In a 16–bit processor, 16–wire bus can carry 16 bits of data, i.e., transfer 2 bytes, etc.

Interaction between CPU, memory and peripheral devices

Address Bus connects CPU and RAM with set of wires similar to data bus. The width of address bus determines the maximum number of memory locations the computer can address. Currently, Pentium Pro, II, III, IV have 36–bit address bus that can address 236 bytes or 64 GB of memory.

Control Bus specifies whether data is to be read or written to the memory, etc.

Expansion Bus

The functions of data bus, address bus and control bus, in the expansion bus, are as follows—

The expansion bus connects external devices to the rest of computer. The external devices like monitor, keyboard and printer connect to ports on the back of computer. These ports are actually a part of the small circuit board or expansion card that fits into an expansion slot on the motherboard. Expansion slots are easy to recognize on the motherboard.

Expansion slots make up a row of long plastic connectors at the back of the computer with tiny copper ‘finger slots’ in a narrow channel that grab the connectors on the expansion cards. The slots are attached to tiny copper pathways on the motherboard (the expansion bus), which allows the device to communicate with the rest of computer.

Data Bus is used to transfer data between I/O devices and CPU. The exchange of data between CPU and I/O devices is according to the industry standard data buses. The most commonly used standard is Extended Industry Standard Architecture (EISA) which is a 32-bit bus architecture. Some of the common bus technologies are—

Peripheral Component Interconnect (PCI) bus for hard disks, sound cards, network cards and graphics cards,

Accelerated Graphics Port (AGP) bus for 3–D and full motion video,

Universal Serial Bus (USB) to connect and disconnect different devices.

· Address Bus carries the addresses of different I/O devices to be accessed like the hard disk, CD ROM, etc.

· Control Bus is used to carry read/write commands, status of I/O devices, etc.