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What does a 4-bit Micro-processor do?

  1. Feb 25, 2013 #1
    Today I just came across a video in youtube,"Making your own 4-bit processor". Does a 4-bit processor performs only addition. I also searched and came to know that Intel 4004 was the first commercially available micro-processor. Did Intel released a processor which worth only adding?
  2. jcsd
  3. Feb 25, 2013 #2


    Staff: Mentor

    The 4-bit micros could be ganged together to make an arbitrarily larger CPU. I think the initial audience was for cash register machines and desktop calculators.

    Doing addition was a great feat. Other arithmetic operations could be done in software, slower but still workable.

  4. Feb 25, 2013 #3

    Simon Bridge

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    A 4-bit microprocessor can do any manipulation that can be done 4 bits at a time.
    All depends on how you encoded it and what it is hooked up to.

    If you only use 4 bits per instruction, then you can have 32 separate instructions.
    If one of the instructions is "treat the following two nibbles as one instruction" then you could, in principle, have 256 additional instructions - each taking 3 machine cycles to load. But an instruction could be "run in 8-bit mode" in which case everything takes two machine cycles to load.

    It depends on how clever the designers were.
    The 4004 had 46 instructions... 41 were 8 bits wide.
    But you can do a lot with 32 instructions.

    Before the 4004, the CPU was not even all one component.
  5. Feb 25, 2013 #4


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    In a way 8 commands are enough to write any program (think brain**** - sigh, profanity filter doesn't allow to properly post name of the language).
  6. Feb 25, 2013 #5


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    The Intel 4004 has an add, subtract, and a decimal adjust instruction (for handling bcd addition, I don't see an adjust for subraction, so you'd have to use 10's complement math for negative bcd numbers (subtract a number from all 9's, then increment the result)). I'm not sure if early 4004 based calculators used binary or bcd.

    Another 4 bit / bit slice processor chip series (not a complete cpu in a chip) was the AMD 2900 series. Several 16 bit mini-computers were based on the 2900 series of chips. Additional hardware was used to convert a mini-computer's machine language into 2900 operations.

    Last edited: Feb 25, 2013
  7. Feb 25, 2013 #6

    Simon Bridge

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    Only require the command set to be Turing complete don't we - something like that?
    Thue is Turing complete, has 1 command and 2 operands.
    ... but most of the implementation is in the hardware in that case right?

    ... and in the good ol' days, we used to have to build out own CPUs one valve at a time. Why the kids of today...
    Last edited by a moderator: Feb 25, 2013
  8. Feb 25, 2013 #7


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    And I though it was hard and nerve-wrecking putting my new i5 into place :biggrin:
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