AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Outline

1 Announcements

2 Kernel Types

3 Simple OS Examples

4 System CallsSystem Call OverviewTypes of System Calls – §2.4, SGG

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Announcements

Week03 labstrace: program to watch system callsC programming tuteICT Learning Centre C programming intro.

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

The OS Kernel

The kernel is the of an OSIt has complete control over everything that occurs in thesystemKernel overview and comparison

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Monolithic Kernel

Older monolithic kernels were written as a mixture ofeverything the OS needed, without much of a structure.The monolithic kernel offers everything the OS needs:

, memory management,multiprogramming, interprocess communication (IPC),

, file systems, network protocols, etc.Newer monolithic kernels have a modular design, whichoffers run-time adding and removal of services. The wholekernel runs in "kernel mode", a processor mode in whichthe software has full control over the machine. Theprocesses running on top of the kernel run in "user mode",in which programs have only access to the kernel servicesModern OS with monolithic kernels: Linux, FreeBSD,NetBSD, Solaris

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Microkernel

Moves as much from the kernel into “user” space to run asprocessesCommunication takes place between user modules using

BenefitsEasier to extend a microkernelEasier to port the operating system to new architecturesMore reliable (less code is running in kernel mode)More secure

DisadvantagesPerformance overhead of user space to kernel spacecommunication

Example microkernels: L4, Minix, QNX

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Monolithic vs. Microkernel System Structure

Monolithic Kernel Microkernel

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Hybrid Kernels

Similar to microkernels, except that they include additionalcode in kernel space so that such code can run moreswiftly than it would were it in user spaceThese kernels represent a compromise that wasimplemented by some developers before it wasdemonstrated that pure microkernels can provide highperformanceHybrid kernels should not be confused with monolithickernels that can load modules after booting (such as Linux)Example OS with hybrid kernels: Microsoft Windows NT,2000 and XP, Windows 7, DragonFly BSD

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

OS Kernels Comparisons

(courtesy Wikipedia)

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Modules

Best current methodology for OS design involves usingOOP techniques to create a modular kernelKernel has set of core components and links in additionalservices as required – either at boot time or run timeUses dynamically loadable modules

Solaris and 7 types of loadable modules

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

Mac OS X Structure

Apple’s Mac OS X uses a hybrid structureTop layers include application environments and servicesproviding a graphical interface to applicationsBelow these layers is kernel environment, which consistsmainly of

Mach microkernel: provides memory management, threadscheduling and support for RPCs and other types of IPCBSD kernel: file system support, networking

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

MS-DOS

Shell invoked when systembootedSimple method to run program:No separate process createdSingle memory space; fig. (a)Loads program into memory,overwriting all but the kernel; fig.(b)On program exit shell isreloaded; back to (a)

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

FreeBSD

Unix variantMultitaskingOn user login, invoke user’schoice of shellShell executes fork() systemcall to create process

Executes exec() to loadprogram into processShell waits for process toterminate or continues withuser commands

Process exits with:code = 0: no errorcode > 0: error code

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Outline

1 Announcements

2 Kernel Types

3 Simple OS Examples

4 System CallsSystem Call OverviewTypes of System Calls – §2.4, SGG

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Overview

System call: a request made by any arbitrary program tothe OS for which the said program doesnot have required permissions to executeSystem calls are generally available as

instructions and they are usuallylisted in the various manuals used by theassembly-language programmers

Mostly accessed by programs via a high-level ApplicationProgram Interface (API) rather than direct system call useManaged by run-time support library (set of functionsbuilt into libraries included with a compiler)

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Overview (contd.)

Three most common APIs are:Windows APIPOSIX API for POSIX-based systems (including virtuallyall versions of UNIX, Linux, and Mac OS X)Java API for the Java virtual machine (JVM)

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Example Use of System Calls

System call sequence to copy the contents of one file toanother file

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Linux open() Example

The Linux open() function – a function (which uses a systemcall) for opening a file prior to reading / writing

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Implementing System Calls

Typically, a number associated with each system callSystem-call interface maintains a table indexed accordingto these numbers

The system call interface invokes intended system call inOS kernel and returns status of the system call and anyreturn valuesThe caller (our program) need know nothing about how thesystem call is implemented

Just needs to obey API and understand what OS will do asa result callMost details of OS interface hidden from programmer byAPI

Managed by run-time support library (set of functions builtinto libraries included with compiler)

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

API – System Call – OS Relationship

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Standard C Library Example

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

System Call Parameter Passing

Three general methods used to pass parameters to the OS

Simplest: pass the parameters in registers...but what if there are more parameters than registers?

Parameters stored in a block, or table, in memory, andaddress of block passed as a parameter in a register

Each individual system call knows how to treat the tablecontentsThis approach taken by Linux and Solaris

Parameters placed, or pushed, onto the stack by theprogram and popped off the stack by the operating system

Block and stack methods do not limit the number or lengthof parameters being passed

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Links to Explore

How to start developing your own OS kernelJava APIPOSIX APIMac OS X API

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Outline

1 Announcements

2 Kernel Types

3 Simple OS Examples

4 System CallsSystem Call OverviewTypes of System Calls – §2.4, SGG

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Categories

Process controlFile managementDevice managementInformation maintenanceCommunications

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Process Control

end, abortload, executecreate process, terminate processget/set process attributeswait for timewait/signal eventallocate and free memory

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

File Management

create/delete fileopen, closeread, write, repositionget/set file attributes

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Device Management

request device, release deviceread, write, repositionget/set device attributeslogically attach or detach devices

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Information Maintenance

get/set time or dateget/set system data

number of current usersamount of free memory(numerous other statistics)

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AnnouncementsKernel Types

Simple OS ExamplesSystem Calls

System Call OverviewTypes of System Calls – §2.4, SGG

Communication

create, delete communication connectionsend, receive messagestransfer status informationattach or detach remote devices

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