The Interrupt flag (IF) is a flag

bit The bit is the most basic unit of information in computing and digital communication. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented as ...

in the CPU's

FLAGS register The FLAGS processor register, register is the status register that contains the current state of an x86 CPU. The size and meanings of the flag bits are architecture dependent. It usually reflects the result of arithmetic operations as well as infor ...

, which determines whether or not the (CPU) will respond immediately to maskable hardware

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 ...

s. If the flag is set to 1 maskable interrupts are enabled. If reset (set to 0) such interrupts will be disabled until interrupts are enabled. The Interrupt flag does not affect the handling of

non-maskable interrupt In computing, a non-maskable interrupt (NMI) is a hardware interrupt that standard interrupt-masking techniques in the system cannot ignore. It typically occurs to signal attention for non-recoverable hardware errors. Some NMIs may be masked, but ...

s (NMIs) or software interrupts generated by the INT instruction.

Setting and clearing

In a system using

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 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

architecture, the instructions CLI (Clear Interrupt) and STI (Set Interrupt). The POPF (Pop Flags) removes a word from the stack into the

, which may result in the Interrupt flag being set or cleared based on the bit in the

from the top of the stack.

Privilege level

In systems that support privileged mode, only privileged applications (usually the OS kernel) may modify the Interrupt flag. In an

system this only applies to

protected mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...

code (

Real mode Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...

code may always modify the Interrupt flag). CLI and STI are privileged instructions, which cause a general protection fault if an unprivileged application attempts to execute them. The POPF instruction will not modify the Interrupt flag if the application is unprivileged.

Old DOS programs

Some old

DOS DOS (, ) is a family of disk-based operating systems for IBM PC compatible computers. The DOS family primarily consists of IBM PC DOS and a rebranded version, Microsoft's MS-DOS, both of which were introduced in 1981. Later compatible syste ...

programs that use a protected mode DOS extender and install their own interrupt handlers (usually games) use the CLI instruction in the handlers to disable interrupts and either POPF (after a corresponding PUSHF) or IRET (which restores the flags from the stack as part of its effects) to restore it. This works if the program was started in real mode, but causes problems when such programs are run in a DPMI-based container on modern operating systems (such as

NTVDM Virtual DOS machines (VDM) refer to a technology that allows running 16-bit/32-bit DOS and 16-bit Windows programs when there is already another operating system running and controlling the hardware. Overview Virtual DOS machines can operate e ...

under Windows NT or later). Since CLI is a privileged instruction, it triggers a fault into the operating system when the program attempts to use it. The OS then typically stops delivering interrupts to the program until the program executes STI (which would cause another fault). However, the POPF instruction is not privileged and simply fails silently to restore the IF. The result is that the OS stops delivering interrupts to the program, which then hangs. DOS programs that do not use a protected mode extender do not suffer from this problem, as they execute in V86 mode where POPF does trigger a fault. There are few satisfactory resolutions to this issue. It is usually not possible to modify the program, as source code is typically not available and there is no room in the instruction stream to introduce an STI without massive editing at the assembly level. Removing CLI's from the program or causing the V86 host to ignore CLI completely might cause other bugs if the guest's interrupt handlers aren't designed to be re-entrant (though when executed on a modern processor, they typically execute fast enough to avoid overlapping of interrupts).

Disabling interrupts

In the

instruction set In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...

CLI is commonly used as a

synchronization Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are sa ...

mechanism in uniprocessor systems. For example, a CLI is used in

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

s to disable interrupts so kernel code (typically a driver) can avoid

race conditions A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events, leading to unexpected or inconsistent ...

within an

interrupt handler In computer systems programming, an interrupt handler, also known as an interrupt service routine (ISR), is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, software ...

. This is necessary when modifying multiple associated data structures without interruption.

Enabling Interrupts

The STI of the

enables interrupts by setting the IF. In some implementations of the instruction which enables interrupts, interrupts are not enabled until after the next instruction. In this case the sequence of enabling interrupts immediately followed by disabling interrupts results in interrupts not being recognized.

Multiprocessor Considerations

The Interrupt flag only affects a single processor. In

multiprocessor Multiprocessing (MP) is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. The ...

systems an interrupt handler must use other synchronization mechanisms such as locks.

References

External links

Intel 64 and IA-32 Architectures Software Developer Manuals
- Retrieved 2017-09-14 {{X86 assembly topics X86 instructions Interrupts