Operating System: Functions and Structure

Operating System Overview

• The operating systems course introduces techniques for implementing operating systems and related system software.
• Will cover operating systems functions and structure.
• Topics include: process management, processor scheduling, deadlock prevention, avoidance, and recovery.
• Main memory and virtual memory management are discussed.
• Also covers the control of disks and other input/output devices.
• File-system structure and implementation, and protection and security are also included.

Introduction to Operating System

• Computers operate in two modes: kernel mode and user mode.
• The operating system runs in kernel mode (supervisor mode).
• Programs in kernel mode have complete hardware access and can execute any machine instruction.
• Other software runs in user mode, with limited machine instructions, restricting control and I/O operations.
• User interface programs (shell or GUI) is lowest level of user-mode software, allows user to start programs.

Operating System as an Extended Machine

• It runs on bare hardware and provides the base for all other software.
• A key difference exists between the operating system and user-mode software.
• Users can choose or create their own email readers.
• Users cannot modify the clock interrupt handler, as it is part of the operating system and hardware-protected.
• Operating systems are large, complex, and long-lived compared to user applications.
• Linux/Windows OS source code can be over five million lines.
• It is software running in kernel mode and performs two main functions.
• Provides application programmers with a clean, abstract set of resources.
• Manages hardware resources.
• The architecture at the machine-language level can be difficult to program, especially for I/O.
• Disk drivers provide an interface to read/write disk blocks without dealing with hardware details.
• The real customers of the operating system are the application programs.
• End users interact with abstractions offered by the user interface.

Operating System as a Resource Manager

• From a bottom-up perspective, it manages the components of a complex system.
• It is responsible for orderly allocation of processors, memories, and I/O devices.
• Resource management includes multiplexing (sharing) resources in time and space.
• Time multiplexing involves different programs taking turns using a resource.
• Space multiplexing involves allocating parts of the resource to different programs.

Computer Generations: Evolution of Operating Systems

• First Generation (1945–1955): Used vacuum tubes.
  • Early digital computers were developed during World War II.
  • Programming was done in absolute machine language or by wiring electrical circuits.
• Second Generation (1955–1965): Used transistors and batch systems.
  • Transistors made computers reliable enough for commercial use.
  • Clear separation emerged between designers, builders, operators, programmers, and maintenance staff.
  • These machines, called mainframes, were housed in large, air-conditioned rooms.
• Third Generation (1965–1980): Used ICs and multiprogramming.
  • Computer manufacturers had distinct product lines.
  • Scientific computers for calculations and commercial computers for data processing.
• Fourth Generation (1980–Present): Personal computers emerged.
  • LSI (Large Scale Integration) allowed personal computers.
  • In 1974, Intel wanted an OS to test the 8080 CPU.

Computer hardware review

• The OS extends the computer's instruction set and manages resources and knows hardware.
• Review of computer hardware includes CPU, memory and I/O devices.
• A simple personal computer has the CPU, memory, and I/O devices connected by a system bus.
• The CPU fetches/executes instructions.
• The basic CPU cycle is fetch, decode, execute, and repeat.
• The program counter has the memory address of the next instruction. -The stack pointer points to the top of the current stack which holds the data.

Memory structure

• Memory should be fast, large but cheap.
• The hierarchy of memory includes: registers, cache, main memory, magnetic disk.
• Higher layers have higher speed, smaller capacity, and greater cost per bit.
• Registers are internal to the CPU and just as fast.
• Cache lines are kept in a high-speed cache close to the CPU using cache hits.
• Disks are cheaper than RAM but slower.
• Disk consists of metal platters that rotate.
• Disk data is written on tracks, tracks from arm positions write on cylinders.
• People talk about non-disk disks in the form of SSD's or solid state drives
• Solid state drives do not have moving parts. so are not really "disks"

I/O Devices and managing the system

• The CPU and memory are resources the OS must manage.
• I/O devices have a controller and the device itself.
• The controller manages the device, and has registers for communication.
• Registers control disk address, memory address, and read/write direction.
• All device register collections form the I/O Port Space

Busses

• As processors and memories become faster the bus becomes strained to the breaking point
• Added additional busses for faster I/O devices and CPU to memory traffic
• The PCI express Buss was invented by intel that is faster then any predecessors at tens of gigabytes/second
• Shared bus architecture means that multiple devices use the same wires, so you need an arbiter to know who can use the bus
• PCI uses parallel bus architecture to send each word of data over multiple wires.
• PCI-e has serial bus architecture and send all bits in a message through a single connection like a network packet
• CPU talks to memory over a fast DDR3 bus to an external device via PCI-e and other devices via a DMI hub.

Booting

• Every PC has a parentboard (motherboard) that contains a system BIOS (Basic Input- Output system)
• the BIOS has a low-level I/O software for the keyboard, screen, I/O disk that is held in flash memory
• when a computer boots. the BIOS does the work, checking Keyboard, and if other devices are installed/responding
• The BIOS also scans PCIE and PCI busses to detect all the devices attached to them.
• BIOS checks for the boot device (CD, USB), if fails then it boots from hard disk.
• Boot data is read into memory and excuted.
• the OS then queries the BIOS to get the configuration information.
• The OS checks for a device driver, if not the OS prompts the user to add one
• the OS loads all device drivers into the kernel.
• The OS then initializes tables, starts background processes and a login program or GUI.