Page "Intel 8080" Paragraph 8

Most 8-bit operations could only be performed on the 8-bit accumulator ( the A register ).

For dyadic 8-bit operations, i. e. 8-bit operations with two operands, the other operand could be either an immediate value, another 8-bit register, or a memory cell addressed by the 16-bit register pair HL.

Direct copying was supported between any two 8-bit registers and between any 8-bit register and an HL-addressed memory cell.

Due to the regular encoding of the MOV-instruction ( using a quarter of available opcode space ) there were redundant codes to copy a register into itself ( MOV B, B, for instance ), which was of little use, except for delays.

However, what would have been a copy from the HL-addressed cell into itself ( i. e., MOV M, M ) was instead used to encode the HLT instruction ( halting execution until an external reset or interrupt ).