The Operating System

Operating System: Lets computers share peripherals and memory, communicate, and run more than one program.

• “The government of your computer”

Kernel: Performs critical system functions and interacts with the hardware.

• Connects system hardware to application software
  • Coordinates all programs running on the computer
  • Interfaces between applications and actual data processing at the hardware level.
• Manages computer resources
  • e.g., Peripherals, files, memory, etc.
• Loaded into memory during the boot process, and always stays in physical memory.

Systems Utilities: Programs and libraries that provide various functions through systems calls to the kernel.

More on the Kernel

Main Kernel Tasks

1. Resource Allocation: Share limited resources among competing processes.
   • Shares CPU, memory, and I/O devices
2. Process Management: Allow execution of applications and support them with features like hardware abstraction.
   • Process: Defines which memory portions an application can access
3. Memory Management: Allow processes to safely access system’s memory.
   • Kernel has full access to the system’s memory.
   • Virtual Addressing: Helps kernel create virtual partitions of memory in two disjointed areas:
     1. User Space: Reserved for applications
     2. Kernel Space: Reserved for the kernel
4. I/O Device Management: Give processes access to peripherals connected to the computer.
   • Device Drivers: Program that enables operating system to interact with a hardware device.
     • Provides how to control and communicate with a certain piece of hardware.
5. Inter-Process Communication: Provides methods for synchronization and communication between processes (IPC).
   • Various methods of IPC: Semaphore, shared memory, message queue, pipe, named FIFO, etc.
6. Scheduling: Gives every program a slice of time and rapidly switches from process-to-process to make it seem like they are being executed simultaneously.
   • Scheduling Algorithms: Algorithms which determine what process to run next and how much time it should be given. Sets priority among processes.
7. System Calls and Interrupt Handling:
   • System Call: Mechanism used by an application to request a service from the operating system.
     • e.g., close, open, read, wait, write
   • Most kernels provide a C library or API to invoke their kernel functions.
8. Security of Protection Management: Provides protection from faults (error control) and from malicious behaviors (security)
   • Language based protection system: One approach to security/protection management, in which the kernel will only execute code produced by a trusted compiler.

Kernel Subsystems

1. Process Management
   • Schedules processes to run on CPU
   • Inter-process communication (IPC)
2. Memory Management
   • Virtual memory
   • Paging and swapping
3. I/O System
   • File system
   • Device drivers
   • Buffer cache

System Calls

System Calls: Set of functions provided to other programs to interface with the kernel.

• Used to request access to the resources of the machine, communicate with other programs, start new programs, etc.
• Grouped by families based on function
  • e.g., Memory management, time management, process management, …

Note: Standards and programming languages
Over the years, standards have been developed to standardize some system calls across UNIX versions, like the ones set by the POSIX committee.

• Many programming languages’ standard library functions use system calls as part of their code.
• e.g., C’s malloc() function actually uses the mmap() and brk() system calls behind the scenes.

Note: The standard C library provides a portion of the system call interface for many versions of UNIX and Linux.

UNIX System Calls

Kernel API: Interface to the kernel.

• Over 1k system calls available on Linux.

Five Categories of System Calls:

1. File Management
   • e.g., mkdir(), open(), read()
2. Process Control
   • e.g., fork(), exit(), nice()
3. Information Management
   • e.g., getuid(), time(), alarm(), sleep()
4. Device Management
   • e.g., ioctl(), read(), write()
5. Communication
   • e.g., pipe(), shmget(), mmap()

UNIX Structural Layout

1. User Space
2. Kernel
3. Devices