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IOPL-sensitive instructions

    Instructions that deal with I/O not only need to be restricted;
    they need to be exectued by procedures that execute at privilege
    levels other than zero. To allow this, the 80286+ uses two bits of
    the flags register to store the IOPL. The IOPL defines the
    privilege level needed to execute I/O related instructions. The
    following instructions can be executed only if CPL is less than
    or equal to IOPL. They are called IOPL-sensitive because they are
    sensitive to the value stored in IOPL.

                                        Protected  Protected
                                           mode     V86 mode
        IN      Input from Port              +
        INS     Input String from Port       +
        OUT     Output to Port               +
        OUTS    Output String to Port        +
        CLI     Clear Interrupt Flag         +         +
        STI     Set Interrupt Flag           +         +
        PUSHF   Push Flags                             +
        POPF    Pop Flags                              +
        IRET    Interrupt Return                       +
        INT     Interrupt                              +
        INTO    Interrupt on Overflow                  +
        LOCK    Assert Bus Lock                        +
        Note  + = IOPL-sensitive, CPL=3 in V86 mode


    To use sensitive instructions, a procedure must execute at a
    privilege level at least as privileged as that stored in IOPL. Any
    attempt by a less privileged procedure to use one of the
    instructions listed produces a general protection exception.
    This allows a V86 monitor to trap - and possibly modify the
    results of - IOPL-sensitive instructions in a program executing in
    V86 mode.


    Each task in the system has its own unique copy of the flags
    register. Therefore each task can have a different IOPL. A task
    can change the IOPL only with a POPF instruction. Such changes
    are privileged. No procedure may alter IOPL in the flags register
    unless the procedure is executing at privilege level 0 (CPL=0).
    Any attempt by a less privileged procedure to alter IOPL does not
    result in an exception, but IOPL remains unchanged.