Back when I was in university, one of the units touching Assembly[0] required students to use subtraction to zero out the register instead of using the move instruction (which also worked), as it used fewer cycles.

I looked it up afterwards and xor was also a valid instruction in that architecture to zero out a register, and used even fewer cycles than the subtraction method; but it was not listed in the subset of the assembly language instructions we were allowed to use for that unit. I suspect that it was deemed a bit off-topic, since you would need to explain what the mathematical XOR operation was (if you didn't already learn about it in other units), when the unit was about something else entirely- but everyone knows what subtraction is, and that subtracting a number by itself leads to zero.

[0] Not x86, I do not recall the exact architecture.