CPU out of logic gates

  1. DrenDran


    Mar 17, 2017
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    CPU out of logic gates

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    DrenDran, Apr 11, 2017
    Arouse1973 likes this.
  2. DrenDran


    Oct 5, 2014
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    Bluejets, Apr 11, 2017
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  3. DrenDran


    Sep 29, 2008
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    Back in the 80's , three of us decided to breadboard a very simple computer using SSI (5 Volt 74xx TTL) integrated circuits for a college course with a lab. This was about the time the 4004 and 8008 processors came out. One of us took the memory, I did the program counter/branching and another did the data structures.

    I learned the "hard way" why the program counter has to point to the NEXT instruction.

    The instruction set was 16 bits x 16 words and what we would call microcoded. It could sort a list of 4 bit numbers on ascending or descending order depending on the microcode.

    Memory is cloudy. I think we had a bit that did "branch", branch on less (A<B). Branch on greater (A>B), swap, load A and load B and probably store A and store B. I just can't remember. That's almost 37 years ago.

    I think the clock may have been a 555 timer. It wasn't fast and it wasn't intended to be.

    The PDP-11 mini-computer schematics were an inspiration. If I remember right, instructions for that machine were microcoded into a 128 bit wide microcode.

    I should still have my lab notes.


    In the same time frame, two of us wrote an OS and machine simulator for a Computer Science class. A large feat for two people. Everyone else had 4 or 5 in the group.


    At home, I wrote code for the 1802 processor. Everything was compiled manually. There is no subroutine instructions in the 1802. It passes the program counter to execute a branch. Any register can become the program counter. The first order of business was to write code that would emulate a subroutine call and return.
    KeepItSimpleStupid, Jul 2, 2017
  4. DrenDran


    Jun 7, 2017
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    I still have 2 or 3 of those, one a Netronics ELFII, and the others home-built, wirewrapped, with extended memory and an EPROM programmer. I suspect that, like most thing electronic, they'll no longer work after sitting around for 30+ years :(

    The KENBAK-1, arguably the first "personal" computer, since you could buy one for only a month or two's salary, was built with logic gates, no microprocessor. Recently a Kenbak-1 sold for about $30,000.
    Irv, Sep 6, 2017
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