I'm taking an assembler course at school and they asked this question:
What are illegal use of the next operations:
1. mov bh,al
2. mov dh,cx
3. mov bh,bh
4. mov cl, 4F2h
5. mov cx, 002dh
6. xchg var1, var2
What are the situations where the operations with mov and xchg are not legal?
Certain combinations of operands are legal, and certain combinations aren't. Most notably, both operands must be of the same size, and at least one operand must be a register. I think those facts combined should suffice to let you determine which of the above are legal and which are not.