INT is an assembly language instruction for x86 processors that generates a software interrupt. It takes the interrupt number formatted as a byte value.^[1]

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

INT X

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.

One of the most useful DOS software interrupts 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.^[2]

Most Unix systems and derivatives do not use software interrupts, with the exception of interrupt 0x80, used to make system calls. 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.

  INT 3

The INT 3 instruction is defined for use by debuggers to temporarily replace an instruction in a running program, in order to set a breakpoint. Other INT instructions are encoded using two bytes. This makes them unsuitable for use in patching instructions (which can be one byte long). (see SIGTRAP)

The opcode for INT 3 is 0xCC, as opposed to the opcode for INT immediate', which is 0xCD imm8. According to Intel documentation: "Intel and Microsoft assemblers will not generate the CD03 opcode from any mnemonic" and 0xCC has some special features, which are not shared by "the normal 2-byte opcode for INT 3 (CD03)" [IA-32 Arch. Software Developer’s Manual. Vol. 2A]

  See also

• INT 10H
• INT 13H
• INT 14H
• INT 21H
• Interrupt
• Hardware interrupt
• BIOS interrupt call
• Ralf Brown's Interrupt List

  References