I've been working on a Game Boy emulator, and I've noticed that there are certain opcodes that exist that would never change any values, such as LD A, A, LD B, B, etc. and also AND A. The first ones obviously don't change anything as they load the value of registers into the same registers, and since the AND is being compared with the A register, AND A will always return A. Is there any purpose for these operations, or are the essentially the same as NOP after each cycle?
As Jeffrey Bosboom and Hans Passant pointed out on their comments, the reason is simplicity. More specifically hardware simplicity.
LD r,r' instructions copy the content of source register (r') to destination register (r). LD r,r' opcodes follow this form:
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BIT | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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OPCODE | 0 | 1 | r | r' |
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Destination and source registers can assume these values:
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| BIT | REG |
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| 111 | A |
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| 000 | B |
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| 001 | C |
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| 010 | D |
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| 011 | E |
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| 100 | H |
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| 101 | L |
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In order to implement these instructions in hardware we just need a multiplexer that receives bits 0-2 to select the source register and another multiplexer that receives bits 3-5 to select the destination register.
If you want to verify if bits 0-2 and bits 3-5 are pointing to the same register you would have to add more logic to the CPU. And as we all know, ressources were more limited in the 80's :P
Please note that loading instructions such as LD A,A, LD B,B, LD C,C, LD D,D, LD E,E, LD H,H, and LD L,L behave like NOP. However AND A and OR A DO NOT behave like NOP, since they affect the flag register, and their execution might change the internal machine state.