See: [https://www.righto.com/2014/10/how-z80s-registers-are-implemented-down.html How the Z80's registers are implemented]
Reverse-engineering the: [https://www.righto.com/2013/11/the-z-80s-16-bit-incrementdecrement.html 16-bit increment/decrement circuit] [https://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-how-it.html 4-bit ALU]
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