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Thank you very much for help!

But why circuit cycles as you described? I'm just pressing "Add" button instantly and by that supplying voltage to Clock only for a moment. So after releasing button loop shouldn't even start, because when Clock becomes 0 latch output doesn't change. Maybe loop starts too fast before I release ...

Hello, guys.
I'm reading Charles Petzold's book "C.O.D.E" and there is an adding machine example that uses 8-bit latch to save the sum for further addition. Here's the circuit image from book: https://i.imgur.com/aVF9Rwp.png.
I've built this circuit in program, but it throws the error "Oscillation ...