stack segment stack 'stack'
      dw 32 dup(?)
stack ends

data segment 

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 bx,1234H
      mov al,bh
      and al,0F0H
      mov cl,4
      shr al,cl
      mov ch,bl
      and ch,0F0H
      mov cl,4
      shr ch,cl
      mov cl,ch
      mov dl,bl
      and dl,00FH
      mov bl,bh
      and bl,00FH

      ret
begin   endp
code    ends
      end begin