ALIGN Directive

The ALIGN directive causes the assembler to align the next data item or instruction on an address according to a given value. The number could be 1, 2, or 4. If it equals 1, the next variable or byte is aligned on a 1-byte boundary. If number is 2, the next variable is aligned on an even-numbered address, and if number is 4, the next address is a multiple of 4.

 ALIGN 1  ALIGN 2  ALIGN 4 
 
 
 .data
 
 
 X  BYTE  0FFh
 
 
 ALIGN 1
 
 
 Y  DWORD  12345678h
 
 
 .code
 
 
 mov   eax, 0
 
 
 mov   ebx, OFFSET X
 
 
 mov   ax, [ebx + 2]
 
 
 call  WriteHex
 
 
 
 .data
 
 
 X  BYTE  0FFh
 
 
 ALIGN 2
 
 
 Y  DWORD  12345678h
 
 
 .code
 
 
 mov   eax, 0
 
 
 mov   ebx, OFFSET X
 
 
 mov   ax, [ebx + 2]
 
 
 call  WriteHex
 
 
 
 .data
 
 
 X  BYTE  0FFh
 
 
 ALIGN 4
 
 
 Y  DWORD  12345678h
 
 
 .code
 
 
 mov   eax, 0
 
 
 mov   ebx, OFFSET X
 
 
 mov   ax, [ebx + 2]
 
 
 call  WriteHex
 Output  Output  Output
   



   



   




†The CPU can process data stored at even-numbered addresses more quickly than those at odd-numbered addresses.

If the offset is already at the required address, it is not advanced. The assembler fills unused bytes with zeros for data and NOPs for instruction. The NOP instruction is used to delete/insert machine code, to delay execution for purposes of timing, or to align a subsequent instruction on a word boundary. NOP simply performs a null operation by executing “XCHG AX, AX” .

 NOP — No Operation 
Usage: NOP

Flag O D I S Z A P C
Result                
Clocks
Operands 286 386 486 Size Bytes
none 3 3 1 1




      “I’m not arguing, I’m just explaining why I’m right.”    
      ― Unknown