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)


   

If the first operand of movl is a memory location, it also moves the contents to the destination register.

 movl -4(%ebp), %eax


   

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

 ; add contents of memory -4(%ebp)  and the contents of  register %eax
 ; store the result as the new contents of the memory location -4(%ebp).      

 addl %eax, -4(%ebp)  


   

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