Structure of Per-Process Data (u.)
Within each process accessed by the kernel exists a unique data structure containing the private variables of the process used to provide UNIX system call functionality. This is called the "extended state" of a given process and is collected into one location. If the process is long inactive, this state is swapped to secondary storage to reclaim RAM memory. All of the machine-dependent fields in this structure lie within the first element u_pcb, a process context descriptor. However, the size of this structure and its adjoining kernel stack is also a machine-dependent parameter. The u. is currently defined at about 1 Kbyte in size. This fits amply within a single page.
Another page is sufficient to hold a kernel stack. This results in a per-process data structure two pages in size. By leaving these as two separate pages in 386BSD, instead of combining them into a single page (giving us a smaller kernel stack), the kernel stack segment can be used to catch the stack overflow ("redstack") condition. This will appear as a future enhancement.
|
|