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Suggestion of Dead ROM bit

To date it has been possible to demonstrate that:

  • the microcode ROM is emitting plausible microcode data
  • the machine initialises correctly by loading microcode from address zero
  • that some execution sequence follows, but this does not result in correct machine operation
  • that single-word microcode show the execution engine is performing at least some functions correctly

This still leaves two general possibilities for the ongoing failure:

  • that the ROM is delivering incorrect microcode (perhaps just one incorrect but critical bit)
  • that there is a specific fault in the execution engine.

Considering these, it seemed simplest to instrument the entire ROM output and assess if the microcode values were in agreement with the available listings.

Suggestion of ROM Fault

Setting up the HP16500C to monitor all ROM address lines and output lines while the machine was attempting to run microcode soon showed a suspicious finding. The 16500C shows a live report of activity on all monitored lines and it can be seen that the 3-bit BI value in the microcode has varying bits 0 and 2 but bit 1 is always zero.

The microcode BI field bit 2 is closely involved in RAM access so a fault in this bit would prevent RAM operations and clearly this would bring down the whole microcode.