What is a Process?

A process is a program in execution. A program is a passive entity (a file on disk); a process is an active entity with a program counter, register state, stack, heap, and a set of resources (open files, memory).

A process consists of:

• Text section — the executable code.
• Data section — global and static variables.
• Heap — dynamically allocated memory.
• Stack — function activation records.
• Program counter & CPU registers.

Process States

Process Control Block (PCB)

The PCB is the kernel data structure that represents a process. In Linux it is struct task_struct.

Fields stored in the PCB:

• Process identification — PID, PPID, UID, GID.
• Process state — new, ready, running, waiting, terminated.
• Program counter — address of the next instruction.
• CPU registers — saved on context switch.
• CPU scheduling info — priority, scheduling queue pointers.
• Memory management — page tables, base/limit registers.
• Accounting — CPU time, time limits.
• I/O status — open files, allocated devices.

Context Switch

When the OS switches the CPU from one process to another it must:

1. Save the current process's registers and program counter into its PCB.
2. Update the previous process's state.
3. Load the new process's PCB state into the CPU.

Context switches are pure overhead — no user work is done — so the OS aims to keep them short.

Process Scheduling Queues

• Job queue — all processes in the system.
• Ready queue — processes ready to run, in main memory.
• Device queues — processes waiting on a particular I/O device.

Schedulers

• Long-term (job) scheduler — selects which jobs to bring into the ready queue. Controls degree of multiprogramming.
• Short-term (CPU) scheduler — picks the next process to run. Invoked very frequently.
• Medium-term scheduler — swaps processes in/out of memory to relieve memory pressure.

Summary

• A process = program + execution state + resources.
• Five canonical states; transitions are driven by the OS or by I/O events.
• The PCB stores everything needed to suspend and later resume a process.