How can I write my own software interrupt? (I am working on writing a small OS, and I would like to do something similar to what DOS does with INT 21h.

For example, how could I get this code:

;Some code
mov ax, bx
mov cx, dx

...To be run every time INT 21h was called?

Thanks.

Check http://board.flatassembler.net/topic.php?p=42174#42174 (I don't know why I'm using CLI when installing the new handler, that's makes sense only when you modify a hardware interrupt vector).

If you are writing an OS, even a small one, you should be familiar with the INTEL documents, the system programming guide in specific. There you will come across an instruction lidt, this can load your new interrupt-table-scheme. This table should content the specific address of all present interrupts. Other than that, be more specific please, or just ask write my code for me.

I don't know if the (interrupt)tutorials are correct...
http://www.osdever.net/tutorials.php?cat=0&sort=1

In protected mode one must first remap the PIC cause INTEL uses the first 32(0-31) for internal procedures.

http://my.execpc.com/~geezer/osd/intr/index.htm
http://www.mega-tokyo.com/osfaq/InterruptsForDummies

See MiniDOS, a 2k Dos clone, With fully commented fasm code.
http://board.flatassembler.net/topic.php?t=5275&start=0

PS: Iet me know if your talking pmode int's.

@Dex4u: I did made that assumption, cause int 21h is already there in rm

Hello log(21,

It still confuses me; Do you want code that does exactly what INT 0x21 does, or you don't know how interrupts can be hailed by the OS.

That's why I asked to specify.