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Invalid Instruction Code

An invalid instruction code is fetched and executed as normal, only that no sequencer will be able of generate control signals out of that code and, consequently, the End Of Sequence signal (EOS) will not be generated.

As a result, the ET count will continue to run until it exceed the permitted maximum value, which is 7. This count is produced by a 4-bits counter IC. In normal circumstances, it is reset earlier by the EOS signal but in this case, it will continue to run further.

As soon as the count gets to value 8 (more significant bit equals 1), the Fail condition is reported.