Basics

Operating Systems (OS) are fundamental software that manage computer hardware and software resources, providing common services for computer programs.

• Act as intermediary between hardware and software
• Handle tasks like memory management, process scheduling, file system management, and device control
• Crucial role in system security, performance optimization, and providing a consistent user experience across diverse computing devices and environments
• Popular OS: Linux, Windows, VMS, AIX, macOS

Machine Instructions

• The instructions that CPUs execute are binary data, usually 1-2 bytes that represent an instruction (opcode), followed by the data needed to execute it
• Machine code consists of a series of these binary instructions
• Assembly: A helpful syntax for reading and writing machine code, making it easier for humans to understand than raw bits

Random Access Memory (RAM)

• Computer’s main memory bank, storing data for running programs
• The CPU fetches machine code directly from RAM, so code cannot run unless loaded into RAM
• Fetch-Execution Cycle: The CPU uses an instruction pointer to fetch the next instruction. After executing each instruction, it moves the pointer and repeats.
  • The pointer is stored in a register (small storage bucket that is extremely fast for the CPU to read and write to)
  • Each CPU architecture has a fixed set of registers (used from storing temporary values during computations to configuring the processor)
• Some instructions can tell the instruction pointer to jump somewhere else instead, or jump different places depending on a certain condition; this makes reusable code and conditional logic possible.

Kernel

• The core of OS, it has near full access to computer’s memory, peripherals, and other resources, and is in charge of running software installed on computer (userland programs)
• When we boot up the computer, the instruction pointer starts at the kernel
• Linux is just a kernel and needs plenty of userland software like shells and display servers to be usable
• Kernel in macOS is called XNU and Unix-like, and the modern windows kernel is called the NT kernel

Privilege Level

• The mode (privilege level or ring) refers to the control level given to a processor

Kernel Mode	User Mode
CPU can execute any instruction and access any memory	Limited instructions, restricted memory and I/O access
Kernel and drivers run in this mode	Applications run in this mode
Processors start in this mode and switch to user mode before executing programs	N/A

• The current privilege level (CPL) is stored in the code segment (CS) register
  • Ring 0: Kernel mode
  • Ring 3: User mode
  • Rings 1 & 2: Rarely used, intended for drivers in some OS
• System Calls: Functions like open, read, fork, and exit that programs use to request services from the OS kernel

Processes

• Processes are OS abstractions, not something CPUs natively understand or track