STI             Set Interrupt Enable Flag            Flags: O D I T S Z A P C
                                                            - - 1 - - - - - -
STI                                                  IO

        Logic   IF ← 1

    STI sets the interrupt-enable flag (IF). After executing the
    instruction following STI, the CPU responds to external
    interrupts, but only if the next instruction allows the interrupt
    flag to remain enabled.

    If external interrupts are disabled and you code STI,RET (such as
    at the end of a subroutine), the RET is allowed to execute before
    external interrupts are recognized. Also, if external interrupts
    are disabled and you code STI,CLI, then external interrupts are
    not recognized because the CLI instruction clears the interrupt
    flag during its execution.

    Non-maskable interrupts are recognized no matter what the state of
    the IF flag.

    Protected mode
    If the current task has insufficient privilege to alter IF, a
    general protection exception is generated.


    Opcode      Format
    FB          STI


    Length and timing
    Operands    Bytes   8088    186     286     386     486     Pentium
    -            1       2       2       2       3       5       7   NP