Under native OS/2, if the statement IOPL=YES is present in the Config.sys file, applications may include Ring 2 segments in which it is possible to execute CLI/STI instruction (disable-enable hardware interrupts), but not IN/OUT.
The OS/2 subsystem of Windows NT allows OS/2 applications to run Ring 2 code segments (no special statement required in the OS/2 C:\Config.sys) but with the following important restrictions:
The OS/2 subsystem will suspend all the other OS/2 applications in the system and all the other threads in the OS/2 process issuing the CLI instructions, until an STI instruction follows. This emulation of CLI/STI instruction is much more costly in run-time overhead than on native OS/2 (where the CPU simply disables external interrupts, which would violate the Windows NT robustness design rules) and also much more costly than semaphore calls. Therefore, when it is possible to modify the OS/2 application, semaphore calls are the preferred way to implement critical sections.
Such instructions will cause a general-protection fault and the application will be terminated.