求大神解答

sseg segment stack
db 80 dup(0)
sseg ends
code segment
assume cs:code,ss:sseg
kbprg proc
push ds
xor ax,ax
push ax
retf
kbprg endp
code ends
end kbprg
这个代码能返回dos界面
但是这个为什么不行？？
sseg segment stack
db 80 dup(0)
sseg ends
code segment
assume cs:code,ss:sseg
kbprg proc
xor ax,ax
mov es:[0],ax
mov word ptr es:[2],ds
jmp dword ptr es:[0]
kbprg endp
code ends
end kbprg