Process Control Block (PCB) in Operating Systems - Step-by-Step Execution

Concept Flow - Process Control Block (PCB)

Process Created

↓

Create PCB

↓

Store Process Info

↓

Process Runs

↓

Update PCB on Events

↓

Process Ends

↓

Remove PCB

This flow shows how a PCB is created when a process starts, updated during its life, and removed when the process ends.

Execution Sample

Operating Systems

PCB = {
  'PID': 101,
  'State': 'Ready',
  'PC': 0,
  'Registers': {},
  'Memory': '0x1000-0x1FFF'
}

This example shows a PCB storing key information about a process like its ID, state, program counter, registers, and memory range.

Analysis Table

Step	Action	PCB Field Updated	New Value	Reason
1	Process created	PID	101	Assign unique process ID
2	Process ready to run	State	Ready	Process is waiting to be scheduled
3	Process starts running	State	Running	CPU starts executing process
4	Program counter advances	PC	1	Next instruction to execute
5	Process interrupted	State	Waiting	Process waits for I/O
6	Registers saved	Registers	{R1=5, R2=10}	Save CPU state
7	Process resumes	State	Running	CPU resumes process
8	Program counter advances	PC	2	Next instruction
9	Process finishes	State	Terminated	Process completed execution
10	PCB removed	PCB	None	Process no longer active

💡 Process ends and PCB is removed from system

State Tracker

PCB Field	Initial	After Step 2	After Step 3	After Step 5	After Step 9
PID	None	101	101	101	101
State	None	Ready	Running	Waiting	Terminated
PC	0	0	1	1	2
Registers	{}	{}	{}	{R1=5, R2=10}	{R1=5, R2=10}
Memory	None	0x1000-0x1FFF	0x1000-0x1FFF	0x1000-0x1FFF	0x1000-0x1FFF

Key Insights - 3 Insights

Why does the PCB state change from Running to Waiting?

When the process needs to wait for something like input/output, the OS changes the PCB state to Waiting to show it is not currently using the CPU (see step 5 in execution_table).

What information is saved in the Registers field of the PCB?

The Registers field saves the CPU's current register values so the process can resume correctly later (see step 6 in execution_table).

Why is the Program Counter (PC) important in the PCB?

The PC shows the next instruction the CPU will run for the process, so saving it lets the OS pause and resume the process accurately (see steps 4 and 8).

Visual Quiz - 3 Questions

Look at the execution_table, what is the PCB State at step 5?

A Waiting

B Running

C Ready

D Terminated

💡 Hint

Check the 'State' column at step 5 in the execution_table.

At which step is the Registers field updated with CPU values?

A Step 4

B Step 6

C Step 8

D Step 10

💡 Hint

Look for when Registers field changes in the execution_table.

If the process never waits for I/O, which step would be skipped?

A Step 7

B Step 9

C Step 5

D Step 10

💡 Hint

Step 5 shows the process state changing to Waiting for I/O.

Concept Snapshot

Process Control Block (PCB):
- Stores process info like PID, state, PC, registers, memory
- Created when process starts, updated during execution
- Tracks process state: Ready, Running, Waiting, Terminated
- Saves CPU context for switching processes
- Removed when process ends

Full Transcript

A Process Control Block (PCB) is a data structure used by the operating system to keep track of all important information about a process. When a process is created, the OS makes a PCB with a unique process ID (PID), its current state (like Ready or Running), the program counter (PC) showing the next instruction, CPU register values, and memory details. As the process runs, the PCB updates to reflect changes like moving from Running to Waiting if the process needs to wait for input/output. The PCB saves the CPU state so the process can pause and resume correctly. When the process finishes, the PCB is removed. This flow helps the OS manage multiple processes efficiently.