OS Ch2 Lecture Notes

Operating System

Software that provides an environment for applications to execute while managing and controlling the underlying hardware.

Operating-System Service

A function provided by the operating system to applications or users, such as program execution, I/O, file manipulation, communication, and error detection.

User Interface

A mechanism through which users interact with a computer, such as a CLI, GUI, touch interface, or batch interface.

Shell

A user-space command interpreter that reads and executes commands by requesting operating-system services. It is normally not part of the kernel.

Command-Line Interface (CLI)

A text-based interface in which users enter commands, such as bash, zsh, PowerShell, or cmd.exe.

Program Execution

An OS service that loads a program into memory, establishes its execution environment, begins execution, handles termination, and reclaims resources.

I/O Operations

Operating-system services that allow applications to communicate with devices such as disks, keyboards, displays, network interfaces, printers, and USB devices.

Why shouldn't applications normally access hardware directly?

The operating system provides controlled abstractions while device drivers handle hardware-specific details, improving safety and consistency.

File-System Manipulation

OS services that allow programs to create, delete, open, close, read, write, and obtain information about files and directories.

Common UNIX file operations

open(), read(), write(), close(), lseek(), stat(), and unlink().

Process Communication

The exchange of information between processes on the same computer or across a network.

Two major process communication models

Shared memory and message passing.

Shared Memory

A communication model in which multiple processes access a common region of memory.

Why does shared memory usually require synchronization?

To prevent race conditions when multiple processes access or modify the same memory.

Message Passing

A communication model in which processes exchange messages rather than directly sharing memory.

Examples of message-passing mechanisms

Pipes, sockets, and message queues.

Error Detection

An operating-system service that detects and responds to problems such as memory errors, disk failures, illegal instructions, network failures, and file-system corruption.

Resource Allocation

The OS determines how CPU time, memory, storage, I/O devices, and network bandwidth are distributed among competing processes.

CPU Scheduler

The operating-system component that decides which ready process executes next.

Accounting

Tracking operating-system resource usage such as CPU time, memory consumption, disk usage, network traffic, process counts, and I/O activity.

Examples of Linux accounting and monitoring tools

top, ps, time, free, and df.

Protection

Determines whether a user or process is permitted to access a particular resource.

Security

Protects the system against unauthorized access, attacks, and other threats.

Examples of OS protection and security mechanisms

Authentication, file permissions, process isolation, memory protection, access control, and encryption.

Command Interpreter

Another name for a shell; a program that reads, parses, and executes user commands.

What usually happens when a shell receives the command ls?

The shell creates or uses a process and launches the ls program rather than performing the directory listing itself.

System Call

A controlled mechanism through which a user-mode program requests a service from the operating-system kernel.

User Mode

A restricted CPU execution mode used by ordinary applications, libraries, and shells.

Kernel Mode

A privileged CPU execution mode used by the kernel and components such as device drivers, the scheduler, and memory manager.

Privileged Instruction

An instruction that normally can only be executed in kernel mode because it could affect the entire system.

Why can't normal user programs execute privileged instructions?

Restricting privileged instructions protects the system from incorrect or malicious programs.

What happens when a system call is made?

The application requests a service, the CPU transfers control to the kernel and enters kernel mode, the kernel performs the operation, and control returns to user mode.

System-Call Interface

The controlled boundary through which user-space programs invoke operating-system kernel services.

API

A programmer-visible, source-level interface that specifies how software requests a service.

System Call vs API

An API is the programmer-visible interface, while a system call is the actual request made to the operating-system kernel.

Is every library function a system call?

No. A library function may operate entirely in user space or invoke one or more system calls.

printf() vs write()

printf() is normally a C library function, while write() is associated with the operating system's system-call interface.

POSIX

Portable Operating System Interface, a standard programming interface used by UNIX-like operating systems to improve source-code portability.

Important POSIX calls in CSI 4337

fork(), exec(), waitpid(), open(), read(), write(), close(), and dup2().

Major categories of system calls

Process control, file management, device management, information maintenance, communication, and protection.

Process-Control System Calls

System calls that create or terminate processes, load programs, wait for state changes, deliver signals, or obtain process information.

Examples of process-control system calls

fork(), exec(), waitpid(), exit(), kill(), and getpid().

File-Management System Calls

System calls used to open, read, write, close, seek within, inspect, or delete files.

Typical file lifecycle

open → read or write operations → close.

Device-Management System Calls

Controlled operations for requesting, releasing, reading, writing, or configuring hardware devices.

Information-Maintenance System Calls

Calls that obtain information such as time, process ID, user ID, file metadata, system information, and resource usage.

Examples of information-maintenance calls

getpid(), getuid(), and stat().

Communication System Calls

Calls that provide interprocess communication through mechanisms such as pipes, sockets, message queues, and shared memory.

Examples of communication system calls

pipe(), socket(), send(), recv(), and shm_open().

Protection System Calls

Calls used to control ownership, permissions, credentials, and user identity.

Examples of protection-related calls

chmod(), chown(), and setuid().

Three common ways to pass system-call parameters

Registers, a memory block, or the stack.

Passing parameters using registers

Arguments are placed directly into CPU registers before the system call.

Advantage of register parameter passing

It is fast.

Limitation of register parameter passing

The number and size of arguments are limited by the available CPU registers.

Passing parameters using a memory block

A register contains the address of a memory structure containing the system-call parameters.

Passing parameters using the stack

System-call parameters are placed on the process stack for the kernel to access.

System Services

Ordinary user-space programs that provide convenient environments for program development and execution. They are not kernel entry points.

Examples of system services

File utilities, editors, compilers, assemblers, linkers, loaders, debuggers, shells, networking programs, monitoring tools, and background services.

Examples of Linux system programs

cp, mv, rm, ls, gcc, g++, gdb, ssh, ps, top, and systemctl.

Three important levels between a user command and the kernel

Application or system program → Library or API → System call → Kernel.

System Program vs System Call

A system program is an ordinary user-space application, while a system call is a controlled request for a kernel service.

Compiler

Translates source code into machine instructions and metadata, usually producing an object file.

Object File

Compiled machine code and metadata that has not yet been combined into a complete executable.

Linker

Combines object files and libraries and resolves references among them to produce an executable.

Loader

Places an executable into memory, maps required code and data, connects dynamic libraries, establishes the execution context, and transfers control to the program.

Program creation and execution sequence

Source code → Compiler → Object file → Linker → Executable → Loader → Running process.

Static Linking

Required library code is copied directly into the executable.

Advantages of static linking

Simpler deployment because fewer external libraries are required at runtime.

Disadvantages of static linking

Larger executables, duplicated library code, and library updates usually require relinking.

Dynamic Linking

Libraries are associated with a program at load time or runtime rather than being completely copied into the executable.

Advantages of dynamic linking

Smaller executables, libraries can be shared between processes, and compatible library updates may not require rebuilding applications.

Linux shared-library extension

.so

macOS shared-library extension

.dylib

Windows shared-library extension

.dll

Why are compiled applications normally operating-system specific?

The executable format, system-call interface, libraries, runtime environment, and ABI must match the operating system.

Linux executable format

ELF.

Windows executable format

PE.

macOS executable format

Mach-O.

ABI

Application Binary Interface; the binary-level conventions that determine how compiled components interact.

What does an ABI define?

Calling conventions, register usage, data layout, executable format, and binary library or system interfaces.

API vs ABI

An API defines how source code calls services, while an ABI defines how already-compiled components interact at the binary level.

Does POSIX guarantee the same compiled binary runs on every UNIX-like OS?

No. POSIX improves source-code portability but does not guarantee binary compatibility.

Ways to improve software portability

Standard APIs, cross-platform libraries, virtual machines, interpreters, and bytecode-based runtimes.

Examples of cross-platform libraries

Qt, SDL, and Boost.

Policy

Determines what decision or choice should be made by the operating system.

Mechanism

Determines how a particular operating-system operation is performed.

Policy vs Mechanism

Policy answers "what should be done?" while mechanism answers "how is it done?"

CPU scheduling policy example

Deciding whether process A or process B should run next.

CPU scheduling mechanism example

Pausing a process, saving its registers, restoring another process's registers, and resuming execution.

Why separate policy from mechanism?

It allows policies to change without redesigning the underlying implementation mechanism.

Common operating-system design goals

Usability, performance, reliability, security, scalability, compatibility, flexibility, and maintainability.

Why can operating-system design goals conflict?

Improving one property may hurt another, such as additional security checks increasing overhead.

What languages are modern kernels primarily written in?

Mostly C or C++, with assembly for architecture-specific and very low-level operations.

Common uses of assembly in an operating system

Boot code, interrupt entry, context switching, and processor-specific operations.

Monolithic Kernel

A kernel architecture in which most operating-system services execute together in kernel space.

Advantages of a monolithic kernel

High performance, fast communication between kernel components, and low IPC overhead.

Disadvantages of a monolithic kernel

A large trusted kernel, kernel bugs can affect the whole system, and the codebase can be difficult to maintain.

Traditional UNIX kernel architecture

Monolithic.

Linux kernel architecture

Largely monolithic with extensive support for loadable modules.