Process: A running program identified by a PID

• Has a virtual CPU
• Has a virtual address space (code & data & execution stack), PC (program counter) pointer
• Set of registers
• Set of OS resources (open files, connections, signals, etc.)

Process Control Block (PCB): Where OS store data about each processes

• State, PC, registers, scheduling priority, memory page tables, resource usage, IO list

Starting Process:

1. Create new process PCB + address space
2. Load executable (“ready”)
3. Dispatch process (”running”)

Process Lifecycle

• Process Lifecycle: New → {(Blocked →) Ready ↔ Running (→ Blocked)} → Exit

State Queues: Queue of processes in different states

Context Switch: Switch cpu core from one running process to another ready process

1. Set to kernel mode
2. Save registers & PC (A) to kernel stack
3. Jump to trap handler
4. Restore registers & PC (B) from kernel stack
5. Set to user mode

Context Switch Causes

• Voluntary: Current process yield()
• Blocked Calls: Current process request a blocked system resource (e.g. read())
• Timer Interrupt: Timer handler decide to switch process

Creation & Termination

Parent Process: Every process is created by a parent (up to PID1 - init)

• fork: Create a subprocess with a copy of the address space
• exec: Replace the code in the current process with new code

Process Destruction: exit() releases all resources

• Stop process → free memory → turn into zombie process → switch.
• Zombie Process: Keep exit code for parent to collect