OFFSET and SEG operators 

OFFSET operator:

The offset operator returns the number of bytes between the label and the beginning of its segment.
Since it produces a 16-bit immediate value, the destination must be a 16-bit operand.
After executing the following instruction, the BX register contains the address of the variable count. BX may thus be used as a pointer to the count variable.

Example 1: The OFFSET operator
	mov	bx, offset count	; Let BX point to count

Example 2: The OFFSET operator
	bList	db  10h, 20h, 30h, 40h
	wList	dw  1000h, 2000h, 3000h
	mov	di, offset bList	; DI = 0000
	mov 	bx, offset bList+1	; BX = 0001
	mov 	si, offset wList+2	; SI = 0006

The SEG operator:

The SEG operator returns the segment part of a label or variable’s address. It is usually used when a variable is in a segment other than the current data segment, such as an external variable.

Example 1: The SEG operator
	MOV AX, SEG My_msg 	;Load AX with the data segment number 
				;where the variable My_msg is declared.
	MOV DS, AX 		;Let DS point to that data segment 	

In the followig example, the variable array is an external variable. The DS register is made to point to the segment that contains the array variable for the temporary duration of the execution of the code that operates on the variable array.

Example 2: The SEG operator

	push ds			; save DS
	mov ax, seg array	; set DS to segment of array
	mov ds, ax		
	mov bx, offset array	; get the array offset
	...			; perform the array processing
				; at this level.
	pop ds			; restore DS