small address-space can not be used in em64t mode ?

Hi, The transparently flexing user address space method which is called as "small address-space" uses segment to implement the protection for small address sapce. But in em64t or amd64 mode, there is not segment (except FS,GS) exists. So it seems the small address-space method can not be used under 64bit mode. Isn't it ? Thanks in advance. Coly