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Thread: 8080 CPU Quirks

  1. #1

    Default 8080 CPU Quirks

    Curious if anyone is familiar with how the INX, DCX, INR, and DCR instructions were internally implemented on the 8080 CPU. An interesting artifact of these instructions is apparent on a full front panel machine with address lights like the Altair 8800.

    It appears the 8080 CPU may use the PC increment logic to implement the INX, DCX, INR, and DCR instructions. The side effect of this is that the address lines are driven for a brief period during the execution cycle by the value of the register or register pair being incremented. This effect can be seen on the front panel address LEDs.

    B, D, and H appear on A15-A8. C, E, and L appear on A7-A0. This is true for both 16 bit and 8 bit increments.

    Note, the effect cannot be seen as a program is single-stepped from the front panel since the address lines are valid (an actual memory address) during the wait state used to stop execution between each single-step.

    Mike

  2. #2
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    Hi All;

    Mike, as a curious side not,, Dr. Suding wrote a program that Could Tell which of the different 8080 Manufactures were plugged into His CPU Board, Based on their differences on what they did or How the answers were different.. Also, a Z-80 showed up as an Intel 8080..

    Unfortunately, I don't have nor have I seen what was in the Code that Dr. Suding wrote..

    THANK YOU Marty

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    That makes sense. Are you aware of the reverse-engineering of the 8080 microcode project? Here.

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    Hi All;

    THANK YOU, Chuck for that Link, I have been looking for something like this for YEARS !!!

    I still would need to transform from Transistors to Gates, I have not looked at the Verilog file, I have to figure out how to open them..

    THANK YOU Marty

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    Marty, you may also be interested in this project. I don't know if the 8080 implementation here is cycle-accurate, but it's interesting nonetheless.

  6. #6
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    Hi All;

    Chuck, WOW !!! I had Never seen or heard of this Project, This might/would be easier for me to Convert, or try out..

    I will take a Quick look at what He has, or what He is doing..

    THANK YOU, so very Much, If You run into any other Projects like this Please let me know..

    I had Downloaded the files from the 6502 Project for the 8080 or was it the 8085, but, I haven't done anything with them so far..

    THANK YOU Marty

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