What you are attempting to do is doable but rather complex - but if you want so... I'd not connect a cable running all the way into LPT of PC _directly_ on '51 buses, though...

The '51 marks you exactly when it fetches code bytes - look at the /PSEN signal, it is supposed to be connected to /OE (or /CS) of code memory. Watch out - there are 2 code fetches per the 12-clock cycle^*! However, if you'd feed NOPs continuously, there would be only one address increment per 12-clock cycle (the same address would be emitted twice). Have a look at the timing diagrams for code fetch in the "bible" and datasheets.


JW

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* there is a an exemption from this when executing MOVX, but that's not important here