Rahul:
First of all stop trying to mix C and Assembly language. This is really just not a good idea. Do C or do assembler. If you must have assembler in a part of a program for performance then put those subroutines in a separate source file and link it to your C code files.

Next please realize that your method of trying to use the timer interrupt as a break switch to long sequences of C code is not going to work. Maybe just maybe an extremely seasoned programmer can find a way to get a kludge like that to work. But I can infer from your question about LJMPing out of a C timer ISR that you are not that seasoned programmer. Please also note that there are probably more than a few ways to re-visit your long code sequence and re-design it so that this "timer break" would not even be needed. In over 30 years of programming I have never seen an instance where it was not possible to restructure code to alleviate code problems like you find your self with.

Try to think outside the box about what you are really trying to acheive. You should consider the possibility to look at how to take the code that needs a quick response to the timer interrupt and place the minimal part of that job in the timer interrupt routine. Save the results of this time critical part of the job into a global variable or data structure. Finally have the ISR set a bit flag that reports to the main line loop that the timer condition has happened and that the results of that timer function are ready to be processed.

You could think of this job as similar to using a stop watch at a sporting event. Have your assistant (the timer ISR) focus on the single minded task of watching for the critical event to happen and push the stop button at the right moment. Meanwhile you (the mainline code) have been busily serving hotdogs at the sideline. After you get done with the current customer your assistant hands you the stop watch and you can take a leisurely moment to note the reading of the event time on the display which you can note into the log book as necessary. Opps, you have another hot dog customer.

Look at the assembler code in this link. There you will see a main loop that simply looks at a bit flag that gets set via the timer interrupt. In this example the timer interrupt is being used to make the main loop do its logic at a certain set pace.

Michael Karas