Currently I am reading up on some documentation on the Zilog Z80 instruction set. There are a few instructions that has one operand that involves the Accumulator, which is denoted as an 'A' in the data sheet, and another operand that takes any of the registers within the CPU. For the purposes of asking this question, I will only focus on the "ADD A, r" instruction.
As taken from one of the Zilog Z80 datasheets, this is what is presented on the instruction:
As seen from the possible registers that can be chosen, register 'A' is one of them.
Register 'A' is also the Accumulator as given here:
My question is, how is it possible to put both operands as the Accumulator? One thing I should also clarify is that there are other instructions that have this same kind of situation.