Hello All,
If i disable round-robin switching in RTX51Tiny

TIMESHARING  EQU  0 ; default is 5 Hardware-Timer ticks
                    ; 0 disables Round-Robin Task Switching

and if i have two tasks in my code then what would be the basis for task switching??

With round-robin ON by default:
In my code i have two tasks and one serial ISR.

I use task 1 for serial I/O and task 2 for displaying on 8 7-segment display, and am struggling to avoid display flickering lest i use os_switch_task() at places where I wait for TXD to occur:

while(tx_flag!=1)
{os_switch_task();} 

And when I download the code into my P89V51RD2 and debug using Hyperterminal(9600,8-bit,no parity,no handshake) i get proper display values; but the display remains blank if i connect it to my machine sending data at the same baud rate.