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Changing Interrupt Vector Table (Create your own interrupt!)
 Author: InternetNightmare


Another thing I want to write tutorial is about changing interrupts. There are two ways you can do that using DOS interrupts and modifying interrupt vector table directly. Both ways are pretty simple, you need to know these DOS interrupts:


What does it do?


AH = 25h

Set interrupt vector

AL – interrupt number to change

DS:DX – pointer to interrupt function

AH = 35h

Get interrupt vector. Gets address of currently set interrupt.

AL – interrupt number


ES:BX – pointer to interrupt

AH = 4Ch

Exits DOS program ;)

AL – exit code (not sure what it does)


It’s pretty simple, just take a look at the sample code here.

The other way to make your own interrupt is to modify interrupt vector table directly. It’s mapped from 0000:0000 to 0000:0400h in memory. The structure is very simple:





Int 0

(Offset 0000)

(Offset 0002)

Int 1

(Offset 0004)

(Offset 0006)

Int 2

(Offset 0008)

(Offset 0010)



So getting interrupt offset is:
mov ax, [intnum*4]

 And segment:
mov ax [intnum*4+2]


mov ax, [intnum*4] ; offset
mov ax [intnum*4+2] ; segment


Well and how to call the interrupt, I think we all know:
int intnum


Everything is pretty simple. NASM soruce code:

DOS interrupt version – here
Direct modifiying of intvec table - here


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