Introduction Lab

8/6/2019 Introduction Lab

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Operating Systems

1 IntroductionLab notes

Course lectured by Prof. Gabriel Kuper

Lab assist. Ilya Zaihrayeu

http://www.dit.unitn.it/~ilya/os.htm


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Ilya Zaihrayeu Operating Systems Course 2

Operating System

Operating System: a program that acts asan intermediary between a user of a

computer and the computer hardwareQ: What are the two main functions of an

operating system?

A: Make user work with

the hardware convenient

Efficientoperation ofthe computer system


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Operating System, contd

Q: What are the five basic services provided by anoperating system?

A: Program execution: the system must be able to

load a program into memory and run it

I/O operations: a running program may require I/O.

This I/O may involve a file or an I/O device. Forefficiency and protection, users usually cannotcontrol I/O devices directly. Therefore, the operatingsystem must provide some means to do I/O

File-system manipulation: programs need to readand writes files. They also need to create and deletefiles by name


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A [contd]:

Communication: there are many circumstances in

which one process needs to exchange informationwith another process. Communications may beimplemented via share memory, or by the techniqueof message passing, in which packets of information

are moved between processes by operating system Error detection: the operating system constantly

needs to be aware of possible errors. Errors mayoccur in the CPU and memory hardware, in I/O

devices, or in a program. For each type of error, theoperating system should take the appropriate actionto ensure correct and consistent computing


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A possible point from which it is convinient to viewan operating system: a set of disassembled system

components and their interconnections System components are:

Process management

Main-memory management

File management

I/O system management

Secondary-storage management

Networking Protection system

Command-interpreter system


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Process Management

Q: What are the five major activities of an operating systemin regard to process management?

Aprocess is a program in execution. A process needscertain resources, including CPU time, memory, files, andI/O devices, to accomplish its task

A:

Creation and deletion of both user and systemprocesses

Suspension and resumption of processes

Provision of mechanisms for process synchronization

Provision of mechanisms for process communication Provision of mechanisms for deadlock handling


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Main-memory Management

Q: What are the three major activities of an operatingsystem in regard to main-memory management?

Memory is a large array of words or bytes, each with its ownaddress. It is a repository of quickly accessible data shared by theCPU and I/O devices. Main memory is a volatile storage device. Itloses its contents in the case of system failure

A: Keep track of which parts of memory are currently being used

and by whom

Decide which processes to load when memory space becomesavailable

Allocate and deallocate memory space as needed


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File Management

Q: What are the five major activities of an operating systemin regard to file management?

A file is a collection of related information defined by itscreator. Commonly, files represent programs (both sourceand object forms) and data

A: File creation and deletion

Directory creation and deletion

Support of primitives for manipulating files and directories

Mapping files onto secondary storage

File backup on stable (nonvolatile) storage media


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Secondary-storage Management

Q: What are the three major activities of an operatingsystem in regard to secondary-storage management?

Since main memory (primary storage) is volatile and too smallto accommodate all data and programs permanently, thecomputer system must provide secondary storage to back upmain memory. Most modern computer systems use disks as

the principle on-line storage medium, for both programs anddata

A: Free space management Storage allocation

Disk scheduling


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Personal Computers vs. Mainframes

Q: List some differences between personal

computer operating systems and

mainframe operating systems

Mainframe is a very large and expensive

computer capable of supporting hundreds, or

even thousands, of users simultaneously. In the

hierarchy that starts with a simple microprocessor

(in watches, for example) at the bottom and

moves to supercomputers at the top, mainframesare just below supercomputers


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Personal Computers vs. Mainframes,

contdA:

Personal computers only allow one user at a time. There isno need for OS to handle with process scheduling andpermission. No timesharing. For example, in DOS, theuser can have as many processes as he wants but thework must be done one job at a time. If a parent processproduces a child, the parent is suspended until its child

finishes its job. No parallelism achieved

Mainframes, on the other hand, allow timesharing andmany users can log on at the same time. The OS must be

able to check process's owners and permission, etc tokeep everything in order. Parallelism can be achievedthrough process switching. The parents and the childrencan work together simultaneously


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Layered System Design

Q: What is the main advantage and

disadvantage of the layered approach to

system design?In the levered approach the

operating system is divided into

a number oflayers (levels),each built on top of lower layers.

The bottom layer (layer 0), is the

hardware; the highest (layer N)

is the user interface. With

modularity, layers are selectedsuch that each uses functions

(operations) and services of

only lower-level layers


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Layered System Design, contd

A: Advantage: modularity. This simplifies debugging

and system verification. The layers are selectedsuch that each uses functions and services of onlylower-level layers. The first layer can be debuggedwithout any concern for the rest of the system,because it uses only the basic hardware (which isassumed to be correct) to implement its functions.Once the first layer is debugged, its correctfunctioning can be assumed while the second layeris worked on, and so on. If an error is found duringthe debugging of a particular layer, we know thatthe error must be on that layer, because the layersbellow it are already debugged


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Layered System Design, contd

A [contd]:

Disadvantage: system calls take longer since

parameters and data may be modified and/or passedat each layer. For instance, for a user program to

execute an I/O operation, it executes a system call

which is trapped to the I/O layer, which calls the

memory management layer, through to the CPUscheduling layer, and finally to the hardware. Thus

each layer adds overheadto the system call and the

net result is a system call that takes longer then one

does on a non-layered system


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DMA and Multiprogramming

Q: On early computers, every byte of data read orwritten was directly handled by the CPU (i.e. therewas no DMA). What implications does this

organization have for multiprogramming?

Direct Memory Access (DMA) is a technique for transferring datafrom main memory to a device without passing it through theCPU. Computers that have DMA channels can transfer data toand from devices much more quickly than computers without aDMA channel can

Multiprogramming is a rudimentary form of parallel processing inwhich several programs are run at the same time on auniprocessor. Since there is only one processor, there can be no

true simultaneous execution of different programs. Instead, theoperating system executes part of one program, then part ofanother, and so on. To the user it appears that all programs areexecuting at the same time


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DMA and Multiprogramming, contd

A:

The prime reason for multi-programming is to

give the CPU something to do while waiting

for I/O to complete. If there is no DMA, the

CPU is fully occupied doing I/O, so there is

nothing to be gained (at least in terms of CPUutilisation) by multi-programming. No matter

how much I/O a program does, the CPU will

be 100 percent busy


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The Shell and the Kernel

Q: What is the purpose of the commandinterpreter? Why is it usually separate from thekernel?

A command interpreteris the part of a computer operatingsystem that understands and executes commands that areentered interactively by a human or from a program. Insome operating systems, the command interpreter iscalled the shell

Kernelis the central module of an operating system. It isthe part of the operating system that loads first, and itremains in main memory. Because it stays in memory, it isimportant for the kernel to be as small as possible while

still providing all the essential services required by otherparts of the operating system and applications. Typically,the kernel is responsible for memory management,process and task management, and disk management


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The Shell and the Kernel, contd

A: The command interpreter is

usually separate from the

kernel because its purposediffers from that of the kernel.The kernel's job is to allowmultiple programs, multiple

users in multi-user systems toaccess the hardware as if eachprogram had its own computer

Another advantage to separating the kernel from thecommand interpreter is that it allows different

command interpreters to be run without kernelmodification. Since the kernel does not include thecommand interpreter, the user can choose whichshell he or she will use


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Traps and Interrupts

Q: What is the main difference between a trap andinterrupt?

An interruptis a a signal informing a program that an eventhas occurred. When a program receives an interruptsignal, it takes a specified action (which can be to ignorethe signal). Interrupt signals can cause a program tosuspend itself temporarily to service the interrupt.Interrupts can be generated by devices, such as a printer

or keyboard, to indicate that some event has occurred.These are called hardware interrupts

Interrupt signals initiated by programs are called softwareinterrupts. A software interrupt is also called a trap or anexception. Traps are usually caused either by an error (forexample division by zero or invalid memory access), or bya specific request from a user program that an operating-system service be performed


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Traps and Interrupts, contd

A: A trap and an interrupt differ with respect to the

sources that generate them. A trap is a software-generated and may be caused either by an error ora user request while an interrupt is a hardware-generated change-of-flow within the system

An interrupt handler is activated to deal with thecause of the interrupt; control is then returned to theinterrupted context and instruction

An interrupt can be used to signal the completion of

an I/O to obviate the need for device polling while atrap can be used to call operating system routinesor to catch arithmetic errors


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Kernel Mode

Q: Which of the following instructions should be allowed only inkernel mode?a) Disable all interrupts

b) Read the time-of-day clockc) Set the time-of-day clock

d) Change the memory map

A:a) Yes. If CPU allows user processes to disable all interrupts,

chaos will occur. Say one user process disables all interruptsand never re-enables it. No other processes, not even OS itself,can work since there will be no clock interrupt to allow OS toswitch from one process to another

b) No. It is not critical enough

c) Yes. Clock is very important and should be allowed to changeby only kernel

d) Yes. Memory mapping should be reserved only for the kernel


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Ilya ZaihrayeuOperating Systems Course 23

References

Andrew S. Tanenbaum, Modern Operating

Systems

Silberschatz Galvin, Operating System

Concepts

Introduction Lab

Documents

Transcript of Introduction Lab