Thanks for your precious answers guys, it was so valuable.
I need your advuse again for that:
I wrote code to semulate receiving data by MC using polling for RI:

mov tmod,#20h
mov th1,#-12
mov scon,#50h
setb tr1
here: jnb ri,here
mov a,sbuf
mov p1,a
clr ri
sjmp here
end
when i tried to debug it i put a value to SBUF manualy through the windo of it and set RI mannualy also to simulate it received the bit of data but when the loop is broken due to RI setting the line
--mov a,sbuf
cause zeros to move to A, is there any problem for setting RI and putting value for SBUF manually or How could i debug It?
Thanks again