If an operand is in memory, an instruction might want to know just the location (i.e., in order to store some value there).

 movl %eax, -4(%ebp)  ; move contents of %eax to memory location -4(%ebp)

 M[-4(%ebp)] = R[%eax]
   

On the other hand, if the first operand of movl is a memory location, it moves the contents to the destination register.

 movl -4(%ebp), %eax

 R[%eax] = M[-4(%ebp)]     
   

An add instruction would need to know both the values stored at the locations of its operands and the location of the second operand:

 addl %eax, -4(%ebp)  ; add contents of memory register %eax to location -4(%ebp).      

 M[-4(%ebp)] = M[-4(%ebp)] + R[%eax]
   

Note: In both cases these instructions need the contents of the first operand!