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Thread: CPU on Molecular Level

  1. #1
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    Default CPU on Molecular Level

    Hello Folks,

    We have talked extensively about computers on a macroscopic level,
    and my question is more on the molecular level. When CPU reads
    the instruction for doing something like reading from memory and
    writing to another location, to Understand the Instruction
    does it have an internal dictionary to compare the instruction to
    or everything is hardwired?

    I hope this layman interpretation is clear....

    ziloo

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    I'm struggling to understand the question.

    What do you mean "compare" and "understand". My computers don't understand a thing any more than a player piano or a coffee maker does.

    Really, I think you need to grab a text on basic computer architecture, starting with logic elements.

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    Before a long journey into the computer architecture books...

    There are many instructions that are fed into the CPU;
    When for example a MOV instruction is fed into the CPU,
    how is it interpreted by CPU to do its move thing. How are
    all these different instructions distinguished?

    ziloo

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    The opcode and any modifier bits are fodder for a simple decoder, or sometimes a PLA. For example, this circuit takes 3 bits and converts to 1 of 8 outputs:



    But many processors are also micro-coded, so there's another layer of instructions on top of this.

    For inspiration, take a look at these

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    Simpler CPU's hardware decode instructions, more complex processors like the intel 80x86 processors use micro code to translate the intruction set into simpler operations that are hardwired.

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    More small CPUs

    In its simplest form, a computer is nothing more than something that obeys the relationship:

    St+1 = f( St, I)

    where St are states varying with time, I is an input, and f is a function represented by the program. You can make an insanely simple processor from some RAM, ROM and a few gates. You don't even need such niceties as a P-counter or ALU.

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    Quote Originally Posted by Chuck(G) View Post

    But many processors are also micro-coded, so there's another layer of instructions on top of this.
    Yes..... "micro-coded" is the word I was looking for! I have been through
    the logic array stuff before, but its implementation in an actual CPU was not
    very clear to me. As I see it now, for the most part the process is
    very much hardwired, but there is a little bit of intelligence in a microcode.

    This is what the wiki says:

    Microcode is "a technique that imposes an interpreter between the
    hardware and the architectural level of a computer".

    Brilliant as always Chuck....Thank you much!
    And thank you David....


    ziloo

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    Here's an example of how an 8080A CPU could be emulated in microcode with AMD am2900 series bit-slice parts. Not light reading.

    http://www.bitsavers.org/pdf/amd/AMPUB-064_8080sim.pdf

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    And for a change, going to the other extreme, the OISC--and its degenerate relative the ZISC. Yes, they have been constructed and do operate as imagined.

  10. #10
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    I like Monte Dalrymple's text on the subject. The title is "Microprocessor Design Using Verilog HDL" and it can be found at http://www.cc-webshop.com/Microproce...0963013354.htm. It's available on Amazon, and is the go-to text on how to design a microprocessor at the "molecular" level. And, bonus of bonuses, it is the Z80 instruction set being implemented.
    --
    Bughlt: Sckmud
    Shut her down Scotty, she's sucking mud again!

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