Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 21h Function 25h
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     For maximum compatibility, this function should be used instead
     of direct editing of the interrupt vector table.
 
     Before an interrupt vector is modified, its original value should
     be obtained with Int 21h Function 35h and saved, so that it can be
     restored using this function before program termination.
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