Understanding CPUs, OS Layers, and Interfaces: A Visual Guide

1. CPU

1.1 CPU and Instruction Set

CPU consists of an arithmetic unit and a control unit.

CPU instructions

Different operations are performed by different execution units provided by the instruction set.

Programmers generally write code targeting the operating system.

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2. Human and Machine

All relationships are illustrated in the accompanying diagram.

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3. OS

3.1 Purpose and Functions of an OS

OS : Operating System

System Call (Syscall)

General purposes and functions of an OS

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3.2 Programming Layers

Hardware Specification : varies across vendors, making APIs inconsistent.

System Calls : few in number but highly refined.

Library Calls : provide higher‑level functionality built on lower‑level calls, typically C/C++ libraries.

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3.3 Instruction Rings

From inside to outside: Ring 0, Ring 1, Ring 2, Ring 3.

Ring 0 contains privileged instructions, executable only by the OS.

Ring 1 and Ring 2 are historically unused.

Ring 3 contains normal instructions used by applications.

Special case example: mkdir /home/test requires kernel permission.

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3.4 Program Execution Modes

User Space (us)

Kernel Space (system space)

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4. UI: User Interface

For an OS, UI is the user interface; for users, UI is the front‑end.

GUI : Graphical User Interface

CLI : Command Line Interface

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5. ABI and API

ABI : Application Binary Interface – defines low‑level interfaces between programs and the OS or other programs.

API : Application Programming Interface – a set of predefined functions that allow developers to use software or hardware capabilities without needing source code access.

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6. Mainstream CPU Architectures

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7. Popular OS Branches

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Original source: https://segmentfault.com/a/1190000004936765