stack segment stack'stack'
      dw 32 dup(?)
stack ends
data segment
X DW 6587H
Y DW 3458H
Z DW ?
data ends
code segment
begin proc far
      assume ss:stack,cs:code,ds:data
      push ds
      sub ax,ax
      push ax
      mov ax,data
      mov ds,ax

      MOV AX,X
      SUB AX,Y
      CMP AX,0
      JB POS
      MOV Z,AX
      JMP EXIT

POS: MOV AX,Y
      SUB AX,X
      MOV Z,AX
EXIT:
      ret
begin endp
code ends
     end begin