Interrupt flag

IF (Interrupt Flag) is a system flag bit in the x86 architecture's FLAGS register, which determines whether or not the CPU will handle maskable hardware interrupts.

The bit, which is bit 9 of the FLAGS register, may be set or cleared by programs with sufficient privileges, as usually determined by the Operating System. If the flag is set to 1, maskable hardware interrupts will be handled. If cleared (set to 0), such interrupts will be ignored. IF does not affect the handling of non-maskable interrupts or software interrupts generated by the INT instruction.

The flag may be set or cleared using the CLI (Clear Interrupts), STI (Set Interrupts) and POPF (Pop Flags) instructions.

CLI clears IF (sets to 0), while STI sets IF to 1. POPF pops 16 bits off the stack into the FLAGS register, which means IF will be set or cleared based on the ninth bit on the top of the stack.

In all three cases, only privileged applications (usually the OS kernel) may modify IF. Note that this only applies to protected mode code. (Real mode code may always modify IF.)

CLI and STI are privileged instructions, which trigger a general protection fault if an unprivileged application attempts to execute it, while POPF will simply not modify the IF flag if the application is unprivileged.

The privilege level required to execute a CLI or STI instruction, or set IF using POPF, is determined by the IOPL (I/O Privilege Level) in EFLAGS. If the IOPL is set to 2 for example, any program running only in ring 0 can execute a CLI. Most modern operating systems set the IOPL to be 0 so only the kernel can execute CLI/STI. The reason for this is that since clearing IF will force the processor to ignore all interrupts, the kernel may never get control back if it is not set to 1 again.

...
Wikipedia