Comment your code to explain to yourself what it is actually doing.

This exercise alone may well make you see the problem yourself.


You should try to give your code a neat, tabulated layout.
The instructions for how to post your code and retain its formatting are here:
http://www.8052.com/faqs/120199

Something like this:

        org   0000h
        sjmp  start1

        org   0003h
        ;ljmp isr_ex0

        org   0013h
        ;ljmp isr_ex1

        org   0030h
start1: mov   sp,#30h

 

etc,...

        ;Set2 
mesg:   db 'MOVING MESSAGE DISPLAY' 
sis:    db '****PRESENTED BY****' 
bro:    db '****GROUP MEMBERS****' 
ack1:   db 'WITH ASSISTANCE FROM'
ack1:   db '      8052.COM'