Wikipedia
IOPL
The IOPL (I/O Privilege level) flag is a flag found on all IA-32 compatible x86 CPUs. It occupies bits 12 and 13 in the FLAGS register. In protected mode and long mode, it shows the I/O privilege level of the current program or task. The CPL (Current Privilege Level) ( CPL0, CPL1, CPL2, CPL3) of the task or program must be less than or equal to the IOPL in order for the task or program to access I/O ports.
The IOPL can be changed using POPF(D) and IRET(D) only when the current privilege level is Ring 0.
Besides IOPL, the I/O Port Permissions in the TSS also take part in determining the ability of a task to access an I/O port.