STI
The STI instruction enables interrupts by setting the IF flag.
One interesting quirk about the STI instruction is that, unlike CLI which has an immediate effect, interrupts are not actually enabled until after the instruction immediately following the STI. One side effect of this could be IF=0, then executing a CLI instruction immediately after an STI instruction means that interrupts are never recognized. The STI instruction sets the IF flag, but interrupts are not checked for until after the next instruction which in this case would be the CLI which takes effect immediately. This behavior exists so a processor that constantly takes interrupts can still make forward progress. See IA-32 manuals for details.
Read more about this topic: Interrupt Flag