- Where is the XOR signal come from? I only see the XOR is the input of the strange "inverter" and M4.

- What is that inverter topology????

- The pMOS M3 is connected to VDD, does that mean when A = 0 the output XNOR is 1?

I use Cadence Virtuoso to simulate the transient analysis and got the wrong waveform, the XNOR is 1 when A = 0 as I predicted.

I've struglling to understand this circuit for days, please someone explain to me. Thank you all

 

M1 and M2 are just dangling on a node. They have no ability to do anything. I think the diagram is missing something.

 

Diagrams for learning purposes are often over simplified.

 

Diagrams for learning purposes are often over simplified.

This isn't a matter of being simplified, there is zero indication of any role that those two transistors play.

 

over simplified = missing components.

 

Okay, I see what you are saying. I've often seen phases like "over-simplified", in a context like this, to mean that while there isn't enough information to fully understand or analyze the circuit, there is sufficient information to just answer the specific question asked.