BUSES, DATA BUS, ADDRESS BUS & CONTROL BUS

BUSES

In big cities mass transit systems move large number of people from one place to their destinations. In the same way computer has a system that move large amount of bits in the form of electrical pulses from a specified source to a specified destination. In computer, the term bus refers that paths between the components of a comuter. In a computer the bus stops are the control unit, the arithmetic and logic unit, internal memory (RAM, ROM and other types of internal memory), and the device controllers that control the operation of the periperal devices. The bus is the common path way through which the processor send/receives data and commands to/from primaty and secondary storage and all input/output peripheral devices. The capacity of a computer bus depends on the number of data lines it contains. A bus iwth 8 lines can transport 8 bits at a time whereas a bus with 16 lines can transport 16 bits at a time. There are two main buses in a computer, the data bus and the address bus. The one that you hear the most about is the data bus, so when people just say 'the bus', they usually mean the data bus.

DATA BUS

The data bus is an electrical path that connects the central processing unit (CPU) memory, and  the other hardware devices on the motherboard. Actually, the bus is a group of parallel wires. The number of wirs in the bus affects, the speed at which data can travel between components, just as the number of lanes on a highway affects how long it takes people to get to their destinations. Because each wire can ransfer one bit at a time, an eight-wire bus can move eight bits at a time. A 16-bit bus can transfer two bytes and a 32-bit bus can transfer four bytes at a time. Buses are designed to match the capabilities of the devices attached to them. So when CPUs could send and receive only one bytes of data a time. As microcomputer technology improved, chips where built that could send and receive more data at once, and bus designes allowed a winder path through which the data could flow.

ADDRESS BUS

Second bus that is found in every microcomputer is the address bus. the address bus is a set to wires similar to the data bus, it connects only the CPU and memory, and all it carries are memory address. The reason that the address bus is important is that number of lines in it determines the maximum number of memory addresses. For example, one byte of data is enough to 300 represent different values. If the address bus could carry only eight bit at a time, the CPU could address only 300 bytes of memory.  Most of the early PC had 20 bit address buses, so the CPU could address only 20 bytes or 1 MB of data. Today, most CPUs have 32 bit address buses that can address 4 GB (over 4 billion bytes) of memory.

CONTROL BUS

From the internal memory unit, the instructions are transferred to the control unit, which directs the activities of all other units. The control unit sends out control commands to all the components of the system through the bus called the control bus. The control unit directs the transfer of data to the ALU from the internal memory unit for the actual processing within the ALU. Under the control of control unit, a two transfer of data results and the instructions occur between the intrernal memory and the secondary storage.

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