[ECOS] Question about DSRs

[Rajeev]

 Hi,
        we are porting the serial driver of the PID board onto our
customized board.  Transmit and Recieve are both in interrupt mode.
When serial_xmt_char or serial_rcv_char are used in the ISR the driver
seems to work well but when xmt_char is put in the DSR ,  the behaviour
is not consistent, mean to say, when i have repeated write calls in a
loop in my app ,the DSR doesnt get the control at all , but if i have
just two write calls not in a loop then the  DSR gets control but it
waits till cyg_user_start exits.
     Can anybody help me out on the exact behaviour of DSRs, like where
to use and where not to. At what point does it get the control?
thanks
rajeev