i have the problem regarding the register values getting corrupted while using interrupt. i know that the registers used in the main program have to be protected using push and pop instructions.
the problem is , the registers used in the interrupt routene is getting corrupted

here is the program using timer0 to genetrate pwm.

pwm: clr et0 ;pulse width generation
clr tr0
setb ea
mov a,#00h
mov r5,#00h

mov tmod,#01h
mov tl0,1ch
mov th0,1dh
setb et0
setb tr0

label : cjne ,#04h,$ ;generate4pulses ;then,go to main program


main program



;loading timer
clr et0
clr tr0
mov 1ch,#00h ;off time low byte
mov 1dh,#0ffh ;off time high byte
mov 1eh,#00h ;on time low byte
mov 1fh,#0ffh ;on time highbyte


ajmp pwm



org 000bh

clr tr0
clr et0

clr a
mov a,r4
cpl a
mov r4,a

;if A=0, then jump to ontime value.
mov p1,a ;if A=1, then jump to off time value

cjne a,#00h,on

mov a,r5
inc a
mov r5,a

mov tl0,1ch
mov th0,1dh

setb tr0
setb et0
reti

on: mov tl0,1eh ;on time low byte
mov th0,1fh

setb tr0
setb et0
reti

here the register values , r5 , 1c,1d,1e,1f are getting corrupted while in the loop. it works well for some time. then it is getting corrupted. please clear it.