汇编语言关于栈段有关问题

assume cs:codesg,ds:datasg,ss:stackdatasg segment    db 'ibm             '    db 'dec             '    db 'dos             '    db 'vax             'datasg endsstack segment    dw 0,0,0,0,0,0,0,0stack endscodesg segmentstart:    mov ax,stack        mov ss,ax   ;这步执行完栈段就不为0了        mov sp,10h        mov ax,datasg        mov ds,ax        mov bx,0        mov cx,4    s:    push cx        mov si,0        mov cx,3    s0:    mov al,[bx+si]        add al,11011111b        mov [bx+si],al        inc si        add bx,16        loop s0        pop cx        loop s        mov ax,4c00h        int 21hcodesg endsend start