Editing INT (x86 instruction) (section)

== Real mode ==
When generating a ''software interrupt'', the processor calls one of the 256 functions pointed to by the interrupt address table, which is located in the first 1024 bytes of memory while in [[real mode]] (see [[interrupt vector]]). It is therefore entirely possible to use a far-call instruction to start the interrupt-function manually after pushing the flag register.

An example of a useful DOS ''software interrupt'' was interrupt 0x21. By calling it with different parameters in the registers (mostly ah and al) you could access various IO operations, string output and more.<ref>[https://www.pcmag.com/encyclopedia/term/int-21 Definition of: int 21]</ref>

Most [[Unix]] systems and derivatives do not use ''software interrupts'', with the exception of interrupt 0x80, used to make [[system call]]s.  This is accomplished by entering a 32-bit value corresponding to a kernel function into the EAX register of the processor and then executing INT 0x80.