Illegal memory reference

An instruction has an operand that refers to a memory location, but a
memory location is not allowed for that operand. For example,

   mov  [bx],[BYTE PTR A]    ;error, can't move from MEM to MEM

Here, both operands refer to a memory location, which is not a legal form
of the MOV instruction. On the 80x86 family of processors, only one of the
operands to an instruction can refer to a memory location.