Address Bus and Data Bus

    

      1. Address Bus

• The address bus consists of 16 wires, therefore 16 bits(Its width is 16 bits).
• A 16 bit  binary number allows 216 different numbers or 32000 different numbers, i.e. 0000000000000000 up to 1111111111111111. Because  memory consists of boxes, each with a unique address, the size of the address bus determines the size of memory, which can be used.
• To communicate with memory the microprocessor sends an address on the address bus e.g. 0000000000000011 (3 in decimal) to the memory.
• The memory selects box number 3 for reading or writing data. Address bus is unidirectional, i.e. numbers only sent from microprocessor to memory not the other way.

       2. Data Bus

• It carries data in binary from between microprocessor and other external units such as memory. Typical size 8 or 16 bits.
• Size determined by size of boxes in memory and microprocessor size helps determine performance of microprocessor.
• The data bus consists of 8 wires. Therefore, 28 combinations of binary digits.
• Data bus is used to transmit data, i.e. information, results of arithmetic, etc. between memory and the microprocessor.
• Data bus is bi-directional.
• Size of the data bus determines what arithmetic can be done.
• Data bus also carries instructions from memory to the microprocessor. Size of the bus therefore limits the number of possible instructions to 256, each specified by a separate number.