V kapitole The R Register je to popsáno: https://worldofspectrum.org/faq/reference/z80reference.htm
Více detailů zde ;-)
http://www.visual6502.org/JSSim/expert-z80.html
https://www.righto.com/2014/10/how-z80s-registers-are-implemented-down.html
https://github.com/gdevic/Z80Explorer