Why is the PC down?

[leetow2003]

I want to move data to B8000H in P-mode,look:

dseg        segment   use16     
gdt         label   byte       
dummy       descriptor <>     
datas       descriptor <0ffffh,0h,12h,92h,0>
datas_sel   =datas-gdt     
datab       descriptor <100h,8000h,0bh,92h,0>
datab_sel   =datab-gdt   
.......
dseg        ends
;
cseg        segment
            assume cs:cseg
;Just now in Protected mode
            mov ax,datab_sel
            mov ds,ax
            mov si,0
            mov byte ptr [si],48h
            mov byte ptr [si+1],7

;Jump to real mode
cseg        ends

when this program run,the letter 'H' can be displayed,but
the pc is down;but I add codes after instruction "mov byte ptr [si+1],7",
mov ax,datas_sel           
mov ds,ax
the PC run well,I want to know why?
If datas' base address is 120000h,when it jump to
real mode,what value is ds?Is DS 1200h?   

[leetow2003]

dseg        segment   use16     
gdt         label   byte       
dummy       descriptor <>     
datas_sel   =datas-gdt     
datab       descriptor <100h,8000h,0bh,92h,0>
datab_sel   =datab-gdt   
.......
dseg        ends
;
cseg        segment
            assume cs:cseg
;Just now in Protected mode
            mov ax,datab_sel
            mov ds,ax
            mov si,0
            mov byte ptr [si],48h
            mov byte ptr [si+1],7

;Jump to real mode
cseg        ends

when I run this program,PC is down,and I change the datab segment
limit,PC run well:
datab       descriptor <0ffffh,8000h,0bh,92h,0>
I want to why?