The init part

Mov ro,#13d ;when decremented to 00 "one sec" is over
Mov r1,#49d Decremented every 1 sec for 50sec
mov Tmod,#01h;(16 bit timer mode)
mov TH0,#00h
mov TL0,#00h
setB TR0

must be move outside the loop.
Read Claus Knudsen post again.
And as Erik said, don't use registers to pass variable between ISR and main program.