The u Area

In addition to the text, data, and stack segments, the operating system also maintains for each process a region called the u area ( user area). The u area contains information specific to the process (e.g., open files, current directory, signal actions, accounting information) and a system stack segment for process use. If the process makes a system call (e.g., the system call to write in the function main in Program 1.1), the stack frame information for the system call is stored in the system stack segment. Again, this information is kept by the operating system in an area that the process does not normally have access to. Thus, if this information is needed, the process must use special system calls to access it. Like the process itself, the contents of the u area for the process are paged in and out by the operating system.

The conceptual relationship of system and process memory is illustrated in Figure 1.9.