Operating Systems Study Website

• Execute user programs correctly
• Make the computer system convenient to use
• Use hardware resources efficiently

The OS manages processes, memory, files, storage, devices, and security. These are the repeated “must know” responsibilities that appear in nearly every intro slide deck.

• The OS is an intermediary between user and hardware.
• The kernel is the always-running core of the system.
• The operating system is interrupt-driven.
• System calls are the protected doorway from programs to OS services.

The processor executes instructions and uses several key registers during execution. The MAR stores the address of the next memory read or write. The MBR stores data coming from or going to memory. There are also I/O-related registers such as the I/O address register and I/O buffer register. Main memory is volatile and is also called primary or real memory. I/O modules connect secondary memory devices, communication equipment, and terminals through the system bus.

User-visible registers include data registers, address registers, index registers, segment pointers, and stack pointers. Control and status registers include the PC, IR, and PSW. Condition-code flags record results such as zero, negative, positive, or overflow outcomes.

Instruction execution happens in two main steps: fetch and execute. The processor fetches an instruction from memory, loads it into the instruction register, executes it, and then repeats this cycle until the normal flow is interrupted.

Interrupts are critical because I/O devices are slower than the CPU. Instead of wasting CPU time checking devices repeatedly, devices interrupt the processor when they need attention. The processor saves context, runs an interrupt handler, and later resumes the appropriate program.

Multiprogramming allows the processor to execute another program while one waits for I/O. Memory exists in a hierarchy where faster storage is smaller and more expensive. Cache stores copies of frequently accessed memory blocks. DMA allows data blocks to transfer directly between a device and memory, reducing CPU overhead.

The operating system controls the execution of application programs and serves as the interface between applications and hardware. Its main objectives are convenience, efficiency, and the ability to evolve.

The OS supports program development, program execution, access to I/O devices, controlled access to files, system access, error detection and response, and accounting.

The OS manages processor time, memory, I/O devices, and storage. The kernel is the portion of the OS that remains in main memory and contains the most frequently used functions.

Systems evolved from manual serial processing to batch systems with monitors and JCL, and then to multiprogramming and time-sharing systems where processor time is shared more efficiently.

The OS depends on memory protection, timers, and privileged instructions. This leads to user mode and system/kernel mode, where only the OS can execute privileged instructions and access protected memory.

This file also covers virtual memory, paging, availability, confidentiality, data integrity, authenticity, fairness in scheduling, and modern OS ideas such as microkernels, multithreading, SMP, distributed OS, and object-oriented design.

A single CPU can switch rapidly among processes, giving the illusion that several are running in parallel. This is called pseudo parallelism.

A process is an instance of an executing program and includes the current values of the program counter, registers, and variables. Each process can be thought of as having its own virtual CPU.

Processes can be created during system initialization, by system calls, by user request, or by batch-job initiation. In UNIX, creation uses fork() and often exec(). In Windows, CreateProcess is used. Termination can happen through normal exit, error exit, fatal error, or being killed by another process.

A process can be running, ready, or blocked. The scheduler moves processes between these states depending on CPU availability and external events.

The OS stores process information in a process table. Each entry is a PCB that contains scheduling info, memory info, file status, and execution state. The file also includes examples like zombies, orphans, and UNIX process-control calls such as waitpid() and kill().

An operating system is system software that acts as an intermediary between the user and computer hardware. It provides an environment for program execution and manages resources.

The main goals are to execute user programs, make the system convenient to use, and use hardware resources efficiently.

The kernel is the always-running core of the operating system. It manages hardware, memory, processes, files, and devices.

User mode is restricted and used by normal programs. Kernel mode is privileged and used by the operating system to access hardware and protected instructions.

A system call is a request from a user program to the OS kernel for a protected service such as file access, memory allocation, or process control.

The OS is called interrupt-driven because it reacts to device events and software exceptions through interrupts instead of constantly polling for them.

A process is a program in execution. It is an active entity that requires CPU time, memory, files, and I/O resources.

The three classic states are running, ready, and blocked.

In UNIX, fork creates a copy of the current process and exec replaces it with a new program. In Windows, CreateProcess does both in one step.

DMA stands for Direct Memory Access. It allows data to move directly between a device and memory with less CPU involvement.

Cache memory is a small, fast memory that stores copies of frequently used main-memory data to improve performance.

Virtual memory allows a process to run even if not all of it is in main memory by using secondary storage as an extension of memory.

The PCB, or Process Control Block, is the OS data structure that stores information about a process such as state, program counter, scheduling info, and memory info.

Main types include batch OS, multiprogramming OS, time-sharing OS, multiprocessor systems, distributed or network OS, real-time OS, and embedded OS.

Linux is important because it is open source, flexible, stable, secure, and widely used in real systems.

• OS definition and goals
• Resource allocator and control program
• Kernel and user mode vs kernel mode
• Booting, BIOS, UEFI, and bootstrap loading
• Interrupts and interrupt handling
• Programmed I/O, interrupt-driven I/O, and DMA
• Cache memory and memory hierarchy
• OS services
• Multiprogramming and time-sharing
• Process model and process states
• fork/exec vs CreateProcess
• PCB and scheduling
• Virtual memory and paging
• Security, protection, and fairness in scheduling