Editing INT (x86 instruction)

{{Short description|Computer assembly language instruction}}
{{More citations needed |date=May 2024}}
'''INT''' is an [[assembly language]] instruction for [[x86]] [[central processing unit|processors]] that generates a [[software interrupt]]. It takes the interrupt number formatted as a [[byte]] value.<ref name="intel-ref">
{{cite book |url=https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf |access-date=2024-05-23 |title=Intel® 64 and IA-32 Architectures Software Developer's Manual |volume=2 |date=March 2024 |publisher=[[Intel Corporation]] |page=3-520}}</ref>

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

:<code>INT ''X''</code>

where <code>''X''</code> 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, <code>[[INT 13H]]</code> will generate the 20th '''''software interrupt''''' (0x13 is nineteen (19) in hexadecimal notation, and the count starts at 0), causing the function pointed to by the 20th vector in the [[Interrupt vector table|interrupt table]] to be executed.

'''INT''' is widely used in [[real mode]]. In [[protected mode]], '''INT''' is a [[Protection ring|privileged instruction]].<ref name="intel-ref"></ref>

== 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.

== INT3 ==
The '''INT3'''&nbsp;instruction is a one-byte-instruction defined for use by [[debugger]]s to temporarily replace an instruction in a running program in order to set a code [[breakpoint]]. The more general '''INT&nbsp;XXh''' 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 INT3 is <code>0xCC</code>, as opposed to the opcode for INT <var>immediate8</var>, which is <code>0xCD <var>immediate8</var></code>. Since the dedicated <code>0xCC</code> 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 <code>0xCD&nbsp;0x03</code> opcode from mnemonics.<ref name="intel-ref" />

== INTO ==
The '''INTO'''&nbsp;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 <code>0xCE</code>, however it is unavailable in x86-64 mode.

== See also ==
*[[INT 10H]]
*[[INT 13H]]
*[[DOS API]]
*[[Interrupt]]
*[[BIOS interrupt call]]
*[[Ralf Brown's Interrupt List]]

==References==
{{reflist}}

[[Category:X86 instructions]]
[[Category:Interrupts]]