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PDP8/F Progress

PS8 Computing
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As part of an event at TNMOC I need to have a working PDP8 for display.  The event has multiple machines from each decade since 1950 solving a similar task - that is producing a list of the terms in the Fibonacci sequence starting with 0 and 1.   So, visual inspection of the machine and all looks OK.   No changes since I last used it more than 18 months ago.  Powered up OK, and I can deposit and retrieve from core.   Quick bit of code to toggle in to increment the accumulator to reassure myself all is ok.   However it failed!  ISZ (Increment and skip on zero) doesn't skip when the value reaches zero.   The DEC manuals are very good and suggested particular chips on the major register control board.  I replaced those suggested and ISZ now works.  Next task is to have at least on serial card working.  I need one for the terminal (an ASR33 for the event) and perhaps another so that I can implement the clever serial-disk server and possibly run OS/8 on the machine.  First attempt and I can toggle in code to send chars out and all is well, but I couldn't receive incoming characters.   Another chip swop and an intense study of the DIP switch settings on the M8650 card and that now behaves.   Using a PC as a serial termina, and having toggled in the RIM loader, I can successfully 'send' in the BIN loader.  BIN loader runs OK and I can send in the first MainDec diagnostic program.   Runs fine!  Hooray!  So, second instruction set diagnostic sent it and sadly that fails.  Falling over on DCA Test 11.  DCA should clear the accumulator and that test fails - but not always.  I can single step through the test without any problem.  The diagnostic also passed the first ten DCA tests.   Each test is a variation of instructions which should always result in a zero'd accumulator.   So, why DCA 11 fails?   That is the next task!


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