MOV Operator

• Copies data from source operand to destination operand

MOV   destination, source

• Cannot copy data from smaller to larger operand. For example if you are going to copy 16-bit data to a 32-bit register, set first the 32-bit register to 0 and use its lower 16-bit part.

.data
count WORD 1 ; 16-bit value

.code
mov ecx,0    ; 32-bit register
mov cx,count ; lower 16-bit part

• But if you use immediate operands (no data labels) and you copy a smaller to a larger operand, it will work. The upper bits of destination will be cleared out (equal to 0).

.code
mov eax,0ffh
; eax = 0x000000ff

• If the source operand is a data label, it copies data from that memory location to the destination operand. To copy only the memory address (and not the data), use offset operator.

mov eax,offset myVal ; copies memory address 0x00B8102B to eax
mov eax,myVal       ; copies data contained in 0x00B8102B to eax
mov eax,[myVal]     ; same as above

• If the source operand is a register and contains a memory address, you must dereference it if you want to get the actual data.

mov esi,offset myVal ; copies memory address 0x00B8102B to esi
mov eax,esi          ; copies memory address 0x00B8102B to eax
mov ecx,[esi]        ; copies data located in memory address 0x00B8102B to ecx

MOVZX (MOV with zero-extended)

• Allows directly moving smaller operand to larger operand by adding 0’s on remaining part (like a padding)

.data
byteVal BYTE 10001111b  ; byte is 8 bits 

.code
movzx ax, byteVal.  ; ax is 16-bit

• This can only be used on the following type of operands

• If higher bits of destination register has contents, those will be overwritten by 0.

MOVSX (MOV with sign-extended)

• Similar to MOVZX
• The destination operand MSB is set to the MSB of the source operand

.data
byetVal BYTE 10001111b

.code
movsx ax, byteVal