Chapter 8: Process Context — Who's Running Your Code

Vol 3: Computer Core Expedition · Chapter 8

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Metadata Card

Attribute	Value
Keywords	Process State Machine, Context Switching, Scheduling, PCB, fork/exec

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Your Progress

"A program on disk is inert. When you run it, the OS creates a process — an active entity with its own address space, file descriptors, and execution state."

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Encounter 1: Process States

New → Ready → Running → Terminated
           ↕        ↓
         Waiting ← I/O

Encounter 2: PCB (Process Control Block)

Each process has a PCB containing:

• Process ID (PID)
• Register state (for context switching)
• Memory management info
• Open file descriptors
• Scheduling priority

Encounter 3: Context Switching

When the OS switches from one process to another:

1. Save current process's registers to its PCB
2. Load next process's registers from its PCB
3. Flush TLB (if page tables differ)
4. Resume execution of new process

Cost: 1-100 μs (thousands of CPU cycles)

Encounter 4: fork() and exec()

• fork(): Creates a near-identical copy of the current process
• exec(): Replaces the current process's memory with a new program

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Verification Checklist

• [ ] Can explain the process state machine
• [ ] Can describe what happens during a context switch
• [ ] Can explain the difference between fork() and exec()

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Chapter 9: Threads and Synchronization