How to handle an interrupt?

StackOverflow https://stackoverflow.com/questions/7803764

  •  25-10-2019
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Question

I have a task to write a program which handles the division by zero exception. I'm having a hard time finding information about that. As I understand, I need to change the 0th entry in interrupt vector table, but how do I do that? I see there is a LIDT instruction, but wouldn't I have to write the whole table then?

Note, I'll be working in 16 bit mode.

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Solution

Assuming that you mean "real mode" by 16-bit mode, the interrupt vector table is simply located starting at linear address 0, see the wikipedia entry. So it's a matter of getting the old 4-byte far pointer from [0000:0000] so it can be restored later (or chained) and overwriting the entry with your own handler.

If you're using DOS you can use INT 21h/AH=25h to set the interrupt vector and INT 21h/AH=35h to retrieve the old entry.

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