What is INT 21h?

Question

Inspired by this question

How can I force GDB to disassemble?

I wondered about the INT 21h as a concept. Now, I have some very rusty knowledge of the interna

Answer 1

It's a "software interrupt"; so not a hardware interrupt at all.

When an application invokes a software interrupt, that's essentially the same as its making a subroutine call, except that (unlike a subroutine call) the doesn't need to know the exact memory address of the code it's invoking.

System software (e.g. DOS and the BIOS) expose their APIs to the application as software interrupts.

The software interrupt is therefore a kind of dynamic-linking.