stack   segment stack 'stack'
        dw 32 dup(0)
stack   ends
data    segment
mydata  db 12H,34H,56H,78H,9AH
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,0001H
        mov dx,ds:[bx + 1]
        xchg dl,dh
        mov ds:[bx + 1],dx

        mov dl,ds:[bx]
        mov dh,ds:3[bx]
        mov ds:[bx],dh
        mov ds:3[bx],dl

        ret

begin   endp
code    ends
        end begin