[Operating System]Chatper1. Introduction

in Dev on Operating System


Introduction

The introduction of operating system.

What is an Operating System?

This is a program that acts as an intermediary between a user of a computer and the computer hardware. The goal is for two : Common user’s perspective, which means that executing user programs and making solving user problems easier, and System’s perspective, which means resource manager and control program. These shorts into two words : Convenience, make the computer system convenient to use, and Efficiency, use the computer hardware in an efficient manner.

Back to the main point, OS uses for

  • OS is a resource allocator.

    • Manages all (physical and abstracted) resources.

    • Physical Resource : CPU, Memory, HDD, Terminal, Network, etc.
    • Abstracted Resource : Task, segment/page, file system, terminal driver, communication protocol, etc.
    • OS decides between conflicting requests for efficient and fair resource use.

  • OS is a control program : Controls execution of programs to prevent errors and improper sue of the computer.

  • No universally accepted definition.

  • The one program running at all times on the computer is the kernel.

    • Everything else is either a system program (ships with the operating system) or an application program.

In this class, we will see how design and implement operating system.

  • Design and Implementation of OS is not “solvable”, but some approaches have proven “successful”.
  • Internal structure of different Operating Systems can vary widely.
  • Start by defining goals and specifications.
  • Affected by the choice of hardware, type of system.
  • User goals and System goals
    • User goals : Operating System should be convenient to use, easy to learn, reliable, safe, and fast
    • System goals : Operating System should be easy to design, implement, and maintain, as well as efficient, flexible, reliable, and error-free.
  • Important principle to separate : Policy and Mechanism.
    • Policy : What will be done?
    • Mechanism : How to do it?
  • Mechanisms determine how to do something, policies decide what will be done.
    • The separation of policy from mechanism is a very important principle, it allows maximum flexibility.
    • Policy is likely to change across places or over time.
    • A general mechanism insensitive to changes in policy would be more desirable.
  • Example : CPU timer construction
    • Policy : How long is the timer to be set?
    • Mechanism : Detailed various timer construction methods.
  • Example : Priority Management
    • Policy : Priority level decision
    • Mechanism : Detailed Control method for handling priority

How a Computer Starts?

Now, let’s see how a computer start-up.

  • Bootstrap program, (or Bootstrap loader) is loaded at power-up or reboot.
    • Typically stored in ROM or EEPROM, generally known as firmware.
    • Initializes all aspects of system, from CPU registers to device controllers.
    • Must know how to load operating system kernel and starts execution.
    • Some computer systems, such as PCs, use two step process.
      • Simple bootstrap loader in BIOS ROM fetches a more complex boot program in boot sector from disk, which in turn loads the kernel.
      • followed on BIOS -> boot sector -> OS location.

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Computer System Organization

In Computer System Organization,

  • One or more CPUs, device controllers are connected through common bus providing access to shared memory.
  • Concurrent execution of CPUs and devices are competing for memory cycles.

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  • General Operation :
    • CPU and I/O devices can execute concurrently.
    • CPU moves data from/to main memory to/from local buffers.
    • I/O moves data from the device to local buffer of controller.
    • Device Controller
      • Each device controller is in charge of a particular device type.
      • Each device controller has usually a local buffer.
      • Device controller informs CPU that it has finished its operation by causing in interrupt.

Interrupt

By the way, what is an interrupt?

  • Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines.
  • Interrupt architecture must save the address of the interrupted instruction.

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  • Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
  • A trap is a software-generated interrupt caused either by an error or a user request. This interrupt is generated by hardware, similar to Exception Handling, for example, divide by zero, segmentation/page fault, system call, etc.
  • An operating system is interrupt driven.

An interrupt has two types :

  • External interrupt : asynchronous, generated by external hardware device.
  • Internal interrupt
    • Fault : save current instruction -> ISR -> return to faulted instruction.
      • ex) page fault, memory space fault
    • Trap : save current instruction -> ISR -> return to next instruction of trapped instruction.
      • ex) system call, exception handling
    • Abort : ISR -> just abort current task because it is critical.
      • ex) divide by zero

So, how handle interrupts?

  • The operating system preserves the state of the CPU by storing registers and the program counter.
  • Determines which types of interrupt has occurred:
    • Vectored interrupt system, which we saw earlier.
    • Polling system, contrary of interrupt system. It determines state of device through handshaking, busy waiting.
      • Pros : fast response, no context switching overhead.
      • Cons : waste of CPU cycle.
      • Requirement : fast I/O device, shorter I/O routine, and should happen rarely.
  • Dedicated code segments determine what action should be taken for each type of interrupt -> called Interrupt Service (Handling) Routine, ISR.

Storages on a Computer

And then, let’s see what storages is composed on a computer.

  • Main Memory : Large storage media that the CPU can access directly.
  • Secondary storage : extension of main memory that provides large nonvolatile storage capacity.
  • Magnetic disks - rigid metal or glass platters covered with magnetic recording material.
    • Disk surface is logically divided into tracks, which are subdivided into sectors. The disk controller determines the logical interaction between the device and the computer.
  • Electronics disks - flash memory.

Storage systems organized in hierarchy. That considered with speed, cost, size, and volatility.

Also, storage system has its cache - copying information into faster storage system. Main memory can be viewed as a last cache for secondary storage.

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Operating System Structure

OS use multi-programming for efficiency.

  • Single user cannot keep CPU and I/O devices busy at all times.
  • Multi-programming organizes jobs (code and data), so CPU always has one to execute.
  • A subset (rest) of total jobs in system is kept in memory.
  • One job is selected and run via job scheduling.
  • When it has to wait (for example, I/O completion), OS switches to another job.
  • Timesharing (Multitasking) is logical extension in which CPU switches jobs, so frequently that users can interact with each job while it is running.
    • Interactive System
      • Response time should be short, typically less than 1 second.
    • Process : Each user has at least one program executing in memory
    • CPU scheduling : If several jobs ready at the same time
    • Swapping : If processes don’t fit in memory, moves them in and out to run.

Process Management

  • A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.
    • One copy of program and several copies of process.
  • Process needs resources to accomplish its task.
    • CPU, memory, I/O, files, initialization data.
  • Process termination requires (total) reclaim of any reusable resources.
  • Typically system has many processes, some operating system is running parallelly on one or more CPUs.
    • Parallelism by multiplexing the CPUs among the processes.

The operating system is responsible for the following activities in connection with process management :

  • Creating and deleting both user and system processes
  • Suspending and resuming processes
  • Providing mechanisms for process synchronization
  • Providing mechanisms for process communication
  • Providing mechanisms for deadlock handling.

Memory Management

Finally, let’s see how OS manage memory.

  • All data in memory before and after processing.
  • All instructions in memory in order to execute.
  • Memory management activities
    • Keeping track of which parts of memory are currently being used and by whom.
    • Deciding which processes (or parts thereof) and data to move into and out of memory.
    • Allocating and deallocating memory space as needed.
  • OS provides uniform, logical view of information storage.
    • File : Abstracts physical properties to logical storage unit
    • Each medium is controlled by device driver (disk drive, tape drive, etc.) They have varying properties : access speed, capacity, data-transfer rate, access method (sequential or random).
  • File-System management do:
    • Files usually organized into directories.
    • Access control on most systems to determine who can access what.
    • OS activities include (in terms of file systems)
      • Creating and deleting files and directories
      • Primitives to manipulate files and directories
      • Mapping files onto secondary storage
      • Back-up files onto stable (non-volatile) storage media



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