10. PCB — Process Control Block

The OS's complete "file" on a process — everything needed to manage, pause, resume, and terminate it. Lives in kernel space; user programs can't access it directly.

Contents:

1. Identification: PID, PPID, UID/GID
2. State: RUNNING, READY, BLOCKED, etc.
3. CPU context: PC, registers, flags (saved/restored on context switch)
4. Memory info: page table pointer, base/limit registers, segment boundaries
5. Scheduling info: priority, CPU time used, start time
6. I/O status: open file table, pending I/O, allocated devices
7. Accounting: page faults, context switch counts

Field PCB (Process) TCB (Thread)
ID	PID	TID
Registers	Yes	Yes
PC	Yes	Yes
Stack pointer	Yes	Yes
Page table	Yes	No (shares process's)
File descriptors	Yes	No (shares process's)

A TCB is smaller and cheaper — that's why thread switches are faster.

Seeing PCBs on Linux: ps aux, cat /proc/<pid>/status, ls /proc/<pid>/fd, cat /proc/<pid>/maps.

---