How to fetch data from TSS?

[leetow2003]

I want to fetch data from TSS,
first I define TSS in GDT:
dseg segment use16
gdt label byte
dummy descriptor <>
demotss descriptor <104,,,89h,>
demotss_sel =demotss-gdt
....
dseg ends

then I define the TSS segment:
DEMOTSSSEG SEGMENT PARA USE16
DW ?,0
DD ?
DW ?,0
DD ?
DW ?,0
DD ?
DW ?,0
DD 0 ;CR3
DD ? ;EIP
DW ?,? ;eflag
DD ? ;eax
DD ? ;ecx
DD ? ;edx
DD ? ;ebx
DD ? ;esp
DD ? ;ebp
DD ? ;esi
DD ? ;edi
DW ?, 0 ;es
DW ?, 0 ;cs
DW ?, 0 ;ss
DW ?, 0 ;ds
DW ?, 0 ;fs
DW ?, 0 ;gs
DW ?, 0
DW 0
DW $ + 2
DB 0FFH
DEMOTSSSEG ENDS

and then I initialize it:
mov bx,16
mov ax,DEMOTSSSEG
mul bx
mov demotss.basel,ax
mov demotss.basem,dl
mov demotss.baseh,dh

at last in p-mode:
mov ax,demotss_sel
LTR ax
mov ax,demotss_sel
mov ds,ax ;===error
mov ax,[2]
but I find when I add the instruction:mov ds,ax
the PC always restart,why?How to correct it?