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32456RE: Mark 8

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  • jeff_s_jonas
    Sep 20, 2013
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      re: the Mark 8


      I found this to be the most useful documentation:

      http://vintagecomputer.net/mark8/RE_Mark-8_Const-NEW.pdf

      (an account on Degnan's server is required).
      Pg 6-8 describe the console switches.

      I'm afraid something came loose. I could view and set RAM for a while, but after a while it kept failing the same way.
      a) trying to load 0x00 into hi addr gave 0x04 (above the 1k ram)
          trying to load 0xff gave 0xF7
      b) trying to load 0x00 into low addr gave 0x0e,
          trying to load 0xff gave ff (yay!)

      Once the 74193s are loaded, "exam" increments them fine.
      I suspect the 74193 chips need to be reseated (open inputs?)
      ALL the chips are socketed using bare clips instead of plastic sockets,
      so perhaps the issue is not to push them too far in?

      My notes on viewing/setting RAM
      1) set run/step to STEP to halt the CPU
      2) set int/jam to JAM for front panel to take control of memory bus

      3) set upper address byte on toggle switches
      press "LAH" to load into upper address register.
      The RAM card is fully populated with 4 by 8 1101 chips for 1k

      4) set lower address byte on toggle switches
      Press "LAL" to load into lower address register

      setting or incrementing the address causes an immediate READ
      and displays the ram on the upper right "mem data" LEDs.

      the "exam" pushbutton increments the memory address
      and diisplays that byte on the "mem data" display

      the "dep" pushbutton
      1) loads the RAM location with the 8 toggle switches
      2) increments the RAM address (both hi and low)
      3) displays the RAM at that address

      That means you can view ram by
      - setting all switches to 0
      - LAH (set upper address byte to 0)
      - LAL (set lower address byte to 0)
        you now see the contents of address 0
      - press "exam" to your heart's delight

      if you want to alter the byte you see,
      just toggle it in and press "DEP".

      There's no decrement / step BACK function,
      so to view the byte you just modified, you must re-enter the address.

      Until we can view and alter the RAM, there's no way to test the CPU with even the simplest program such as setting port 0 with various values (port 0 has LEDs on the front panel). I tried "run" mode and the LEDs flickered as the CPU was fed 0xff from RAM or running off the end of memory (addresses don't wrap).

      -- jeffj

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