Note:
There are two bits in the flags register which are documented by Zilog as being unused. But in fact these bits do change: these bits are updated according to an undocumented register of the Z80 named either WZ or MEMPTR. This undocumented register is used to store temporary results such as 16-bits arithmetic results, and address (for example, loading I register and the vector byte for interruption mode 2 to compute the value to load in PCregister).
== Sign flag ==