INT is an assembly language instruction for

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

processors A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, ...

that generates a

software interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...

. It takes the interrupt number formatted as a

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

value. 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). As is customary with machine binary arithmetic, interrupt numbers are often written in hexadecimal form, which can be indicated with a prefix ''0x'' or with the suffix ''h''. For example,

INT 13H 




INT 13h is shorthand for BIOS interrupt call 13 hex, the 20th interrupt vector in an x86-based (IBM PC-descended) computer system. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector-based hard disk and  ...

will generate the 20th ''software interrupt'' (0x13 is the number 19 -- nineteen -- written in hexadecimal notation, and the count starts with 0), causing the function pointed to by the 20th vector in the interrupt table to be executed.

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 An interrupt vector table (IVT) is a data structure that associates a list of interrupt handlers with a list of interrupt requests in a table of interrupt vectors. Each entry of the interrupt vector table, called an interrupt vector, is the addre ...

). 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.Definition of: int 21
/ref> Most

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

systems and derivatives do not use ''software interrupts'', with the exception of interrupt 0x80, used to make

system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

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.

INT3

The INT3 instruction is a one-byte-instruction defined for use by

debugger A debugger or debugging tool is a computer program used to test and debug other programs (the "target" program). The main use of a debugger is to run the target program under controlled conditions that permit the programmer to track its executi ...

s to temporarily replace an ''instruction'' in a running program in order to set a code

breakpoint In software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause. More generally, a breakpoint is a means of acquiring know ...

. The more general INT XXh instructions are encoded using ''two'' bytes. This makes them unsuitable for use in patching instructions (which can be one byte long); see

SIGTRAP Signals are standardized messages sent to a running program to trigger specific behavior, such as quitting or error handling. They are a limited form of inter-process communication (IPC), typically used in Unix, Unix-like, and other POSIX-compli ...

. The opcode for INT3 is 0xCC, as opposed to the opcode for INT immediate8, which is 0xCD immediate8. Since the dedicated 0xCC opcode has some desired special properties for debugging, which are not shared by the ''normal'' two-byte opcode for an INT3, assemblers do not normally generate the generic 0xCD 0x03 opcode from mnemonics.

INTO

The INTO instruction is another one-byte-instruction. It is a conditional ''interrupt'' which is triggered when the overflow flag is set at the time of executing this opcode. This implicitly indicates interrupt #4. The opcode for INTO is 0xCE, however it is unavailable in x86-64 mode.

References

{{reflist X86 instructions Interrupts

Real mode

INT3

INTO

See also

References