slides 2 operating systems

8/3/2019 Slides 2 Operating Systems

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OperatingSystems

Luis Moura e SilvaEmail: [email protected] de Eng. InformticaUniversidade de Coimbra

2011/2012

2. Introduction to O.S.


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Disclaimer

This slides and notes are heavily based on the companion material of[Silberschatz05].The original material can be found at: http://codex.cs.yale.edu/avi/os-book/os7/slide-dir/index.html

In some cases, material from [Stallings04] may also be used. Theoriginal material can be found at:

http://williamstallings.com/OS/OS5e.html

The respective copyrights belong to their owners.


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Whats an Operating System?


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What Operating Systems Do:

Sharing resources among applications

scheduling allocation

Making efficient use of limited resources improving utilization minimizing overhead improving throughput/good put

Protecting applications from each other enforcement of boundaries


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Classification of Operating Sytems

Virtual Time and Real Time Operating Systems

Examples of OS with Virtual Time: Windows, Unix, Linux, Mac OS X, Novell,

Examples of Real-Time Operating SystemsVxWorks, VRTX, pSOS, LynxOS,

Embedded Operating Systems Symbian, WindowCE, PalmOS, iPhone OS, Android,

Open/Closed Operating Systems: Proprietary Open systems Open-source systems (GPL license)


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Market Share Desktop OS: July 2011


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Share Mobile Operating Systems: Q1 2011


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Simple Organization of an Operating System

KERNEL

System Libraries

Applications

Protection

Barrier


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Two Modes of Operation (to assure security)

User Mode

Kernel Mode

UNIX Operating System Structure


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Input/Output versus CPU Usage

I/O devices and the CPU can execute concurrently Each device controller is in charge of a particular device Each device controller has a local buffer CPU moves data from/to main memory to/from local

buffers

I/O is from the device to local buffer of controller

Device controller informs CPU that it has finished itsoperation by causing an interrupt


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Interrupt Timeline


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Different Types of I/O


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Device Status Table


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Direct Memory Access (DMA)

Used for high-speed I/O devices able to transmitinformation at close to memory speeds

Device controller transfers blocks of data from bufferstorage directly to main memorywithout CPUintervention

Implies the capability ofarbitrating the system bus

Only one interrupt is generated per block, rather than theone interrupt per byte


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DMA (2)


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Protection

Dual Mode of OperationUser Mode and Kernel Mode In User Mode only common instructions can be performed (e.g.

arithmetic and logic). In Kernel Mode can do anything.

A mode bit contains the current mode of operation RTI: Return from interruption Kernel mode assumed on traps and on interrupts

I/O ProtectionAll I/O Instructions are privileged.

For doing I/O a trap into the operating system must be generated.

Memory Protection Each program can only access its memory

CPU Protection The CPU must remain in control of the Operating System, even if

the applications dont want to let go.


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Dual Mode of Operation


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System Calls, Traps and Interrupts

OperatingSystem

Interrupts

TrapsSystem calls


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Structure of a System Call


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Example with a Hello World Application!

The strace program is very useful: It allows you to see what system calls are made and with what

parameters!

#include

int main() {printf(Hello World\n);return0;

}


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strace in Action!


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Storage Hierarchy

Caches, Buffers and a Storage Hierarchy are Essential


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Storage Hierarchy

A great part of this hierarchy is controlled by theOperating System Cache Management Policies Buffer Management Policies Memory Management Policies

Principles of Locality:

- Temporal Locality

- Spatial Locality


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Memory Protection

Each program must only be able to access its ownmemory Segmentation Virtual Memory

Segmentation is based on having a base pointer and alimit for each process runningAny access made outside of its space generates a trap and

normally leads to the process being killed

Virtual Memory is based on having a table that translatesvirtual addresses into real ones

Normally, there is such a table for each process Each process sees all the address spaceAn access made to an non existing page generates a trap and

normally leads to the process being killed


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Segmentation


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Virtual Memory

0 0

4Gb 4Gb

Address Space

of Process AAddress Space

of Process B

1000 1000

Physical Memory0

256Mb

5000

Address Translation

Table

Disk

(simplified)


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Multiprogramming

Multiprogramming:maintaining several jobs inmemory, active at the same time, so that when thecurrent program can no longer run (for example becauseits blocked waiting for I/O), another is available to run

Optimizes resource utilization: CPU and I/O

CPU

Time Multiplexed CPU

E.g. Windows 95/98!


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Multitasking

Multitasking is an extension of multiprogramming inwhich the execution of the active programs is time-sliced: Each program runs for a short period of time, then another is run. When a program is running and is forcibly replaced by another,

typically with a higher priority, it is said to have been preempted,thus the term Preemptive Operating Systems


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Multitasking

For implementing multitasking, a non-maskableinterrupt must connected to a hardware timer Every few milliseconds (e.g. 100ms), the interrupt causes a

task (or process) switch

total = 0;

for (int i=0; i


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Nowadays

Windows XP, Linux 2.6.xx: Preemptive multitasking operating systems using virtual

memory

Each process thinks it has the whole computer for itself Virtual Machine


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Operating System Architecture and Structures

Currently, there are two dominant architectures

Microkernel: Moves as much as possible from the kernel into user space Communication takes place between user modules

using message passing

Benefits: Easier to extend a microkernel Easier to port the operating system to new architectures More reliable (less code is running in kernel mode) More secure

Detriments: SLOW: Performance overhead of user space to kernel space

communication


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Currently, there are two dominant architectures

Monolithic: Consists of everything below the system-call interface and above

the physical hardware

Provides the file system, CPU scheduling, memory management,and other operating-system functions; a large number of functionsfor one level

Benefits: FAST!

Detriments: Less reliable (more code is running in kernel mode) Less secure No so easy to port to new architectures ( it depends)


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Most modern operating systems implementkernel modules Uses object-oriented approach Each core component is separate Each talks to the others over known interfaces Each is loadable as needed within the kernel

Benefits Its easier to extend the operating systemAllows to isolate functionality in well defined software entities,

making the OS more maintainable and reliability


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Virtual Machines

Facet A:Abstract Machine that runs bytecode (e.g. Java, MS.NET) Good for portability, allowing running the same unmodified

application in different physical architectures

Facet B: Software that allows different operating systems to be run in

parallel giving them the illusion of having a whole machine(e.g. VMWARE, XEN, Virtuozzo, )

Some Virtual Machines run on top of the hardware: IBM z/VM Others run on top of the host OS: XEN, VMWARE, etcAllows for server consolidation, software testing, software fault

tolerance, etc.


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VMWARE Architecture


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Reference

Chapter 1: Introduction 1.1, 1.2, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9

Chapter 2: Operating Systems StructuresAll chapter 2! 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 2.10

slides 2 operating systems

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