The OS/2 subsystem is implemented as a protected server; OS/2 applications communicate with the subsystem by using the local procedure call (LPC) message-passing facility. The subsystem and each application run in their own protected address spaces, which protects them from other processes running with Windows NT.ss
Figure 16.2 OS/2 Subsystem in Windows NT
In native OS/2, applications run in user mode (Ring 3) and communicate with the OS/2 kernel by using calls to the DLLs. Some application programs and DLLs contain I/O privilege segments and are allowed to perform I/O operations in Ring 2. The OS/2 subsystem will attempt to run such programs but those using the I/O privilege to perform IN/OUT instructions (to access some hardware device) violate the robustness features of Windows NT and thus will be terminated with a general protection fault. For more information, see "I/O Privilege Mechanism" later in this chapter.
Figure 16.3 Native OS/2