Operating Systems

Chapter 1 and 2

Memory

• RAM (Random Access)

  • Computer’s temporary workspace

  • Volatile

• SSD/Storage

  • Long-term storage

  • Stores programs/files

  • Nonvolatile

Operating System Services

• User Interface

  • CLI, Touch screen, GUI

• Program Execution

  • Loads a program into memory and run that program, end execution, either
    normally or abnormally (indicating error)

• I/O Device

  • A running program may require I/O, which may involve a file or an I/O device

• Resource Allocation

Main Memory

Only large storage media that the CPU can access directly

CPU

• The primary component that executes instructions and processes information

• Performs instructions such as:

  • Add numbers

  • Compare values

  • Move data

  • Load value from memory

CPU Time

The amount of time a CPU spends processing instructions for a given task or operation.

I/O Devices

• Hardware that allows the computer to receive information from or send information to the outside world

• Example Input:

  • Keyboard

  • Mouse

  • Touch screen

• Example Output:

  • Monitor

  • Speakers

  • Printer

• Example Input and Output:

  • Touch screen on a phone

Multi-tasking

• The OS switching between multiple process very quickly by giving each process little CPU time to make it seem like each process is happening at the same time

Multi-programming

• The technique of keeping multiple programs in memory so the CPU can switch to another one when the current one can’t continue using the CPU

• Example:

  • Task A: running but then needs to read from the disk

  • Task B: uses CPU while Task A is reading

User-mode

• Normal programs that have restricted access to the computer

• Has to ask the OS to do things for it

Kernel mode

• Privileged mode where there’s access to protected hardware/system resouces

Trees in OS

• Stores a hierarchical structure like the file system

Linked list in OS

• If the kernel manages many processes and needs to remove one without shifting the entire list or adding a node without shifting the list

Bitmap in OS

• Stores a lot of on/off information so could be used by the kernel to determine which memory blocks are available

Hashmap in OS

• Used to find a process quickly to get information

Operating System (OS)

• A program that acts as an intermediary between a user and the computer hardware

• Controls and coordinates use of hardware among various applications and users

• Parts:

  • Kernel

  • System Utilities

  • User interface

Memory Management

• Keeping track of which parts of memory are currently being used and by whom

• Deciding which processes and data to move into and out of memory

• Allocating and deallocating memory space as needed

Device Management

• Assigns hardware devices (like printers or disk drives) to active processes when requested and safely releases them when tasks finish

• System calls:

  • request device, release device

  • get device attributes, set device attributes

  • logically attach or detach devices

Kernel

• It is the core component of an operating system that manages system resources and allows communication between hardware and software.

• Manages things like:

  • Who gets CPU time

  • Who gets RAM and where

  • Starting, stopping, and managing processes

  • File systems

  • System calls

  • Security

File Management

Process Management

Security and Protection

Interrupts

• To signify that operations have been completed and to tell the CPU that

  other parts of the operating system need attention.

• Examples:

  • Pressing a key on the keyboard

  • Clicking the mouse

  • Timers that interrupt processes to give another process CPU time

Example of services provided to users/programs by OS

System calls

• A controlled way for a user-mode program to request a service from the kernel

• Examples:

  • Creating/terminating processes

  • Opening/Reading files

  • Requesting memory

Tightly coupled

• Core components of OS are interdependent on other components and are in one address space

• Changes to one part could impact other parts

Loosely coupled system

• OS design relied on autonomous, modular components that interact through well-defined communication interfaces

• Slower but easier to debug

Monolithic kernel

• Kernel has little or no structure, such as layers and modules

  • Very fast because all components are in same address space

• The kernel functionality is placed in a single static binary file in a single address space
  

• Provides the file system, CPU scheduling, memory management, and other operating-system functions

Micro kernel

• Move nonessential components from the kernel into user space to make the kernel as small as possible

• Communication takes place between user modules using message passing

• Advantages:

  • Smaller kernel is easier to port to new architectures

  • More reliable and secure (less code is running in kernel mode)

• Disadvantages:

  • Performance overhead of user space to kernel space
    communication

Layered kernel

• The operating system is divided into several layers (levels), each built on top of lower layers.

• The bottom layer (layer 0), is the hardware

• The highest (layer N) is the user interface
  

• Implementation details are hidden from higher level layers

Separating mechanism from policy

What languages are preferred for OS implementation