INT (x86 instruction)

INT is an assembly language instruction for x86 processors that generates a software interrupt. It takes the interrupt number formatted as a byte value.

When written in assembly language, the instruction is written like this:

where X is the software interrupt that should be generated (0-255).

Depending on the context, compiler, or assembler, a software interrupt number is often given as a hexadecimal value, sometimes with a prefix 0x or the suffix h. For example, INT 21H will generate the software interrupt 0x21 (33 in decimal), causing the function pointed to by the 34th vector in the interrupt table to be executed, which is typically an MS-DOS API call.

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.