Process Table

The Process Table contains an entry for each process present in the system. The entry is created when the process is created by a Fork system call.

Maximum Entries = 16

TICK (1 word): Keeps track of how long the process was in memory/swapped state. It has an initial value of 0 and is updated whenever the scheduler is called. The tick is reset to 0 when a process is swapped out or in.

PID ( 1 word): Process descriptor, a number that is unique to each process. The pid is set to the index of the entry in the process table.

PPID(1 word): PID of the parent process. PPID of a process is set to -1 when it's parent process exits (Orphan process).

USERID (1 word): UserID of the currently logged in user, this field is set by Fork system call.

STATE(2 words): A two tuple that describes the current state of a process.

SWAP FLAG(1 word): Indicates if the process is swapped or not. The process is said to be swapped if any of its user stack pages or its kernel stack is swapped out.

INODE INDEX(1 word): Pointer to the Inode entry of the executable file, which was loaded into the process's address space.

INPUT BUFFER(1 word): Buffer that used to store the input read from the terminal.

MODE FLAG(1 word): Used to store the system call number if the process is executing inside a system call. It is ste to -1 when the process is executing the exception handler.

USER AREA SWAP STATUS: Indicates whether the user area the user area of the process has been swapped (1) or not (0).

USER AREA PAGE NUMBER(1 word): Page number allocated for the user area of the process.

KERNEL STACK POINTER(1 word): Pointer to the top of the kernel stack of the process. The offset of this address within the user area is stored in this field.

USER STACK POINTER(1 word): Logical address of the top of the user stack of the process. This is used when the process is running in kernel mode (Doubt)

PTBR (1 word)

PTLR (1 word)

The Per-Process Resource Table is stored in user area of each process, generally the resource table is stored somewhere else in the memory and the pointer is kept in process table entry.

The Process Table is present in page 56 of the memory

PROCESS_TABLE+PIDx16