After further debugging, my monitor for the Apple 1 with MC6800 microprocessor now works, at least on the MESS simulator. I’ve used it to examine and modify memory, and to run a small test program in RAM.

The monitor still needs to be tested on real hardware, but now I have high confidence that it will work. The monitor is exactly 256 bytes long, the maximum that will fit in the two 256×4 bipolar PROMs used in the Apple 1.

I tried to record video of the session with MESS’ built in avi capture feature. I get an avi file that is hundreds of megabytes, and I haven’t been successful at transcoding it. The worse problem, though, is that the video is rotated horizontally, so that what should be at the left edge of the video is actually almost at the very right edge, and wraps around.

I guess I’ll have to use a camcorder to capture a session.

The assembly source code is here, and the assembly listing is here.

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6 Responses to Success!

  1. Michael says:

    Hi Eric,
    Do you have any plan to make some PROMs based on your 6800 monitor and test if it works in hardware? I have shared your assmebly source code and listing to HobbyROMs and they can manage to make HI and LO images. I hope to test it in my Apple 1 replica using 6800 processor.

  2. Eric says:

    I do plan to test it eventually, but I haven’t finished assembling my Apple I replica yet. I have a suitable PROM programmer.

    It would be great if you tested it. Remember that the Apple I needs to have different clock driver components installed to use the 6800.

  3. Michael says:

    Yes, I will test it and will publish the result, I have a couple of AMI 6800s to test with. Could you give some ideas which ICs exactly needed to be replaced? and by what? I don;t see much information from A1 schematics diagram other than “6800 only” needs to be populated. Thanks.

  4. Eric says:

    Note that the schematics of the 6800 section don’t quite match the actual board:

    You have to remove the two “6502″ jumpers, Add the 7404 at C1, and Q1-4, C8-13, and R16-23.

  5. Michael says:

    I have got the two PROMs based on your source code for some months, today, I soldered the 6800 only area, plugged the 7404, switched 6800 CPU and desoldered the two 6502 jumpers. But no luck seeing the flashing slash… did you try in your replica yet?


  6. Lionel Theunissen says:

    Hi Eric,

    I’ve been playing around trying to get a 6800 processor running on a Apple/Mimeo-1 using your monitor program and after troubleshooting a couple of issues have had success.

    I’ve put a video up:

    It looks like the 6800 clock generator design doesn’t quite work as is. At least I couldn’t get it to work using a 7404 in position C1. However, I found a 74L04 works. I think the 7404 is too fast and the slower 74L04 has enough propagation delay to make it work.

    Also, pin 39 of the processor is left floating and this causes problems with the 6800 processor properly fetching the reset vector after rest. I pulled it to ground with a 10k resistor.



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