Z80 was a popular 8-bit processor with a 4-bit ALU.Implementing a zero flag for a register should be straight forward, it would be a logical NOR of all the bits on the register.Something like that would work for a small number of inputs. As for a 64-bit processor you cannot make one gigantic NOR gate with 64 inputs. The fan-in would be too high. 8 transistors would be in series. The circuit capacitance would be high thus slowing down everything else.I can see some other options.The zero flag could be generated directly from the 8-bit result using 2 level logic.The zero flag could be generated directly from the 8-bit result using 3 level logic.The zero flag could be generated from each nibble and then put together, like if there was a half-zero flag. The result for the lower would be saved using a flip-flop while waiting for the high nibble result to be calculated.Ken Shirriff wrote a nice article about reverse engineering the Z80 ALU. However when it comes to the zero flag he states:Not shown in the block diagram are the simple circuits to compute parity, test for zero, and check if a 4-bit value is less than 10. These values are used to set the condition flags.So, although they are simple circuits I would like to know exactly how they were implemented and if they used any of the implementations proposed above or something else completely different.
How was the Zero Flag implemented on Z80 ALU?
hardware
Well if you want to know how it was done exactlyThen download the Z80 die shot of model you want to investigate, crop the ALU part and identify all the gates you can until you dig to Zero flag your self (sorry for indirect answer).Here my Z80 ALU post processed die shotwhite - metalgreen - poly-Sired   - dopped-Si (diffusion)Gray  - conductive joints between layersTry to identify all gates and buses you can (and mark them to the image)When found familiar structure like Wire OR, (N)OR cascade, ... then you will know for sure. Just try to find the basic components like:form the circuit schematics and make some sense of it.