In SX Microcontrollers, SX/B Compiler and SX-Key Tool, James Newton wrote:

You might be interested in looking at the bock diagram of some microcontroller or microprocessor designs. One of the things you notice is that the logic unit that does stuff like inc, dec, shift, swap, negate, etc... has to be connected in-line with the registers and accumulator wether it is used or not. It doesn't take any more time to turn it on while you are moving data than it does to leave it of. So why not provide opcodes that allow that to happen? It turns out that it takes more logic to turn OFF the ALU for op codes that don't use it than it does to just "hardwire" the bits in the opcode that select the ALU operation to the ALU and let them be active for a wider range of operations. 

I'm not sure I said that in a way that will inc or dec everyones confusion counter but in general, I'm saying that it would probably require more logic in the chip to NOT provide those "move with operation" instructions than it does to just let them happen.

CPU design is fun.

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