Source: Operating Systems: Three Easy Pieces (OSTEP) — Chapter 4, Arpaci-Dusseau & Arpaci-Dusseau

The Process Abstraction

A program on disk is lifeless — a pile of instructions and maybe some static data. A process is that program running: the OS loads it, gives it an address space, and lets it execute. Your laptop seemingly runs dozens of processes at once even with a few CPUs. How?

Crux: how does the OS provide the illusion of many CPUs?

Answer: virtualize the CPU with time sharing — run one process, stop it, run another. The cost is performance: each process may run slower when the CPU is shared.

Mechanisms vs Policies

	Question	Example
Mechanism	How?	Context switch — stop one process, start another
Policy	Which?	Scheduler — which ready process runs next

Separating them is a classic OS design rule: you can change the scheduling policy without rewriting the context-switch machinery.

Time sharing vs space sharing

Time sharing — take turns with a resource (CPU, network link).
Space sharing — divide the resource (disk blocks assigned to files).

What Is Process State?

A process is summarized by its machine state — everything it can read or update while running:

Piece	Role
Address space	Code, static data, heap, stack in memory
Registers	Including PC (next instruction) and stack/frame pointers
I/O info	Open files, etc.

Process creation (high level)

1. Load code + static data into the address space (eagerly in simple OSes; lazily with paging in modern ones)
2. Allocate and initialize the stack (locals, args for main)
3. Allocate a small heap (grows via malloc / OS)
4. Set up default I/O (e.g. stdin / stdout / stderr on UNIX)
5. Jump to main and transfer the CPU to the new process

Quiz

Rank the sequence

1 / 1

Order the steps the OS takes to create and start a process.

OrderDrag the handle to rearrange — top should be first.

1. ⋮⋮1Allocate and initialize the stack
2. ⋮⋮2Load code and static data into the address space
3. ⋮⋮3Jump to main and transfer the CPU to the new process
4. ⋮⋮4Allocate a small heap
5. ⋮⋮5Set up default I/O (stdin/stdout/stderr)

Drag items into the correct order (or use ↑↓), then check.

Process API (ideas every OS exposes)

• Create — start a program (shell command, double-click)
• Destroy — kill a runaway process
• Wait — block until a process finishes
• Control — suspend / resume
• Status — runtime, current state, …

Process States

In the simplified model:

State	Meaning
Running	Currently executing on a CPU
Ready	Runnable, but the OS chose someone else
Blocked	Waiting for an event (e.g. disk I/O) — cannot run until that event

Transitions:

• Scheduled: Ready → Running
• Descheduled: Running → Ready
• I/O initiate: Running → Blocked
• I/O done: Blocked → Ready

Extra states in real systems (xv6 and UNIX): EMBRYO / creating, ZOMBIE (exited but not yet wait()ed by the parent).

Interactive: state transitions over time

Step through two classic traces from OSTEP: CPU-only processes vs a process that blocks on I/O so another can run.

Try:

• CPU only — Process0 runs to completion, then Process1. CPU stays busy.
• CPU + I/O — when Process0 blocks, Process1 runs. When I/O completes, Process0 becomes Ready — the scheduler may keep Process1 running (a policy choice).

Key Insight

Blocking on I/O is what lets the OS keep the CPU useful. Without switching on I/O wait, the CPU would sit idle while the disk works.

Quiz

Rank the sequence

1 / 1

A process starts Ready, gets scheduled, issues an I/O request, and the I/O later completes. Order the states it passes through (its first state first).

OrderDrag the handle to rearrange — top should be first.

1. ⋮⋮1Blocked (waiting for the I/O)
2. ⋮⋮2Running (executes, then issues I/O)
3. ⋮⋮3Ready (again, after I/O completes)
4. ⋮⋮4Ready (initial)

Drag items into the correct order (or use ↑↓), then check.

Check your understanding

Quiz

Processes and CPU virtualization

1 / 4

Time sharing the CPU primarily creates…

Pick an answer to see if you are right.

Coding challenges

Same ideas as process-run.py, in miniature — Python to match that homework’s language.

Homework lab: process-run.py

Grab the OSTEP CPU scheduling homework (process-run.py). Predict each answer before peeking with -c / -p. Use the lab below for notes and screenshots — everything is stored in this browser only (IndexedDB), including pasted images.

Lab

Homework: process-run.py

0/8 started

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Data structures: the process list

The OS tracks every process in a process list (task list). Each entry is a PCB (process control block / process descriptor): pid, state, parent, open files, and a saved register context (so a stopped process can be resumed). xv6’s struct proc and struct context are the textbook picture of this.

When a process exits, it may sit in zombie until the parent wait()s — then the OS frees the PCB.

Summary

Term	Meaning
Process	Running program + machine state
Time sharing	Alternate CPU use → virtual CPUs
Mechanism / policy	How to switch vs which to run
Running / Ready / Blocked	Core states + I/O and schedule transitions
PCB / process list	OS bookkeeping for every process

Next: Process API — fork, exec, wait, and how shells use them.

Further Reading

• Remzi & Andrea Arpaci-Dusseau — Operating Systems: Three Easy Pieces Ch. 4
• xv6 — process structures in proc.h / proc.c