The POSIX subsystem is implemented in Windows NT as a protected server. POSIX applications communicate with the POSIX subsystem through a message-passing facility in the Executive known as a Local Procedure Call (LPC).
Figure 17.5 POSIX Subsystem in Windows NT
The POSIX subsystem and each POSIX application run in their own protected address space, which protects them from any other application that might be running on Windows NT. POSIX application are preemptively multitasked with respect to each other, and with respect to other applications running in the system.