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what is address bus and instruction bit in 8051

Discussion in 'Microcontrollers, Programming and IoT' started by vead, Mar 7, 2014.

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  1. vead


    Nov 27, 2011
    I always confused actually what is 8 bit address bus and instruction . bus is wire or cable that is used to carry the signal
    following is confused term for me
    1) address bit and instruction bit is same or different things?
    2)data bus - if the data is 8 bit and 8bit address it means it can be carry 8 bit data. I am not sure that I am correct or wrong ?
    3) If the control bus is 8 bit then It will carry 8 bit signal I am not sure?
    4) what is address and address bit?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    What do you know about microprocessors? Maybe it's time to get to know the basics before you start designing your own microprocessor.

    The answers to your questions in part depend on the processor architecture.

    The 8015 follows a Harvard architecture.

    1) There is no such thing as an instruction bus in the 8051.
    There are two address busses: One from the program counter to address locations within program memory, one to address data memory.
    2) The width of the data bus adn the address bus are independent. An 8 bit data bus can carry 8 bit data. An 8 bit address bus can address 2^8 memory locations. In general a memory location addressed by n-bits can hold an m-bit data word. "n" and "m" don't have to be the same. The 8051 for example has 16-bit addresses but 8-bit data words.
    3) Any 8-bit bus can carry 8 bits, that's by definition. I don't know which "cotrol bus" you're referring to.
    4) The adddress is the full address of a locatioon, made up by the single address bits. Comparable to a house number where the number is the address, the digits are the single "bits" of the house number.
  3. vead


    Nov 27, 2011
    Data bus is electrical path that connect cpu , memory and other device
    data bus= electrical path
    data bits = number of wires in electrical path
    each wire can transfer 1 bit at time
    we can say 8 bit data bus means 8 wire of 8 bit at time

    address bus- If the address bus is 8 bit It can be done 256 operations
    1 and logic
    2 nor logic
    3 additions
    .256 subtraction (just for example)

    Is it correct?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I', sorry, but that is complete nonsense. An address bus doesn't do "operations". It is, as the name suggests, the means to send an address to a device (RAM, ROM, I/O, ...). An address is used to identify a location within that device, e.g. a memory word, an I/O-port etc. Just as a house number addresses an individual house within a street.

    It look like you're confusing the address bus with the concept of commands (operation code, short opcode). With 8 bit commands you can distinguish 2^8=256 diffferent commands.
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