"paging in interrupt service rourines I am unable to get this concept."
Using 128k of RAM make you have 2 'pages' of 64k each, p1.0 being your 'page switch'

"some details will help me.Also "mirror"bit concept"
This - a mirror bit being a bit that ALWAYS has the same state as p1.0 - will allow you to change the page in an interrupt to whichever page the data you need data from in the interrupt, while being able to restore the page used by 'main' when returning from an interrupt.

My GUESS is that the garbage characters come from either the interrupt returning to main with the wrong page selected or that the ISR reads from the wrong page because it does not select the page inside the ISR.

Erik