Ceng 334 - Operating Systems 6-1

Chapter 6 : Case Studies

• UNIX

• Dos

• Windows 95

• Windows NT

Ceng 334 - Operating Systems 6-2

UNIX

• There are two main versions:

– AT&T System V Release 4 (SVR4)

• Originally developed by AT&T, now SCO

– BSD (Berkeley Software Distribution)

Ceng 334 - Operating Systems 6-3

• Supports various architectures

• Structure varies

• Supports preemptive multitasking

• Multiuser environment - generally secure

• Supports multithreaded applications

• Protection/Security is high on modern versions

Ceng 334 - Operating Systems 6-4

• Supports symmetric multiprocessing• Highly scalabe/portable to various systems• Many types/flavours of UNIX exist

Ceng 334 - Operating Systems 6-5

Ceng 334 - Operating Systems 6-6

MS-DOS

• Limited to x86 architecture

• Simple Structure

• Single User (No tasking, scheduling, priorities)

• No protection

• Monolithic structure

• Old, outdated but still widely used (hardware and cost driven decision)

Ceng 334 - Operating Systems 6-7

Ceng 334 - Operating Systems 6-8

Windows 95• x86 architecture

• Layered

• Single user

• 32 bit applications run in a preemptive, multithreaded, multitasking environment

• 16 bit applications (of Windows 3.x) run in shared memory space in a cooperative multitasking environment

Ceng 334 - Operating Systems 6-9

• Virtual DOS Machine for DOS applications

• “Plug and Play” - new OS “advantage”

Ceng 334 - Operating Systems 6-10

Ceng 334 - Operating Systems 6-11

Ceng 334 - Operating Systems 6-12

Windows 95 ArchitectureSystem Virtual Machine

Win32 app

Win32 app

Win32 app

Systemservices,

KERNEL,USER,

andGDI

Win16 app

Win16 app

DOS app

DOS app

DOS app

DOS Virtual Machines

Ring 3

Ring 0

Virtual Machine Manager subsystem File Management subsystem

Ceng 334 - Operating Systems 6-13

• VMM (Virtual Machine Manager) and file management subsystems run in ring 0 (the most privileged level of the four-ring Intel 386 protection model). This implies these two modules have unrestricted access to the whole system.

• VMM provides memory management, scheduling and DOS Protected Mode Interface (DPMI)

• VMM also includes all Virtual Extended Drivers (VxDs) to virtualize hardware devices.

Ceng 334 - Operating Systems 6-14

• The system Virtual Machine runs in ring 3 (The only other protection level used by Win 95. Ring 3 is the least privileged, best isolated level)

• All applications and core dynamic link libraries (DLLs) - KERNEL, USER, and GDI execute in the VM

Ceng 334 - Operating Systems 6-15

Multitasking & Multithreading

• Multiple concurrent threads

• Time is allocated on a per-thread basis

• DOS VMs and VM address space shared by Win16 applications constitute one thread each

• Each Win32 apps represent at least one, and possibly two or more, threads

Ceng 334 - Operating Systems 6-16

Scheduling

• The scheduler allocates time based on execution priority, taking into account factors such as whether the thread belongs to a foreground or background process and how long it has been since the thread was last run

• Win32 and DOS apps are preemptively multitasked.

• Win16 apps are cooperatively multitasked within the time slice allocated.

Ceng 334 - Operating Systems 6-17

Desynchronized Input Queues

• An input queue is where hardware generated messages such as mouse clicks and keystrokes are stored for app retrieveal

• Win95 assigns each application its own input queue

• In Windows 3.x there is only one input queue so a misbehaving process may delay all other applications

Ceng 334 - Operating Systems 6-18

(Symmetric Multiprocessing) SMP

• Not applicable to Win95.

• Only in NT versions

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File System

• Windows 3.x Oss use DOS services to perform file I/O.

• Win95 uses a 32-bit (16-bit for Windows 3.x) file system VFAT (Virtual FAT)

• VFAT is compatible with FAT volumes

• VFAT permits long file names (up to 255 characters)

Ceng 334 - Operating Systems 6-20

Windows NT

• 32 bit preemptive multitasking OS• Supports different chip architectures (CISC

and RISC)• Can work as a server or client (workstation)• Has the same GUI as Windows 95• Modular style• Multiuser, multitasking• Supports threads as well as classical

processes

Ceng 334 - Operating Systems 6-21

• Support SMP (Symmetric Multiprocessing) (1 to 32 processors)

• Designed with security in mind

• Supports distributed processing - networking is a core component to the OS

• Supports RPCs

• Very robust (reliable)

Ceng 334 - Operating Systems 6-22

• The HAL (Hardware Abstraction Level) makes porting very easy

• Supports non-NT applications through environment sub systems - meaning it can run Win32, OS/2 and POSIX applications (supports multiple personalities)

• NT style OSes are the way of the future

Ceng 334 - Operating Systems 6-23

Ceng 334 - Operating Systems 6-24

Comparative OS Characteristics

Operating System Characteristics Windows95

Windows NTworksation

Preemptive multitasking – 32-bit apps . .Preemptive multitasking – 16-bit apps .Supports multithreaded apps . .Apps run in protected memory space 32 bit only .Subsystems run in protected memoryspace .Supports symmetric processing .Desktop (object-oriented) user interface . Version 4.0

Ceng 334 - Operating Systems 6-25

References

• Modern Operating Systems, Andrew S. Tanenbaum, Prentice-Hall, 1992

• Operating Systems (Second Edition), William Stallings, Prentice-Hall, 1995

• Local Area Networks : A Client/Server Approach, James E. Goldman, John Wiley&Sons, 1997

• Your Next Operating System, PC Magazine, September 26, 1995, pp. 102-159

• Windows 95 And Its Competitors, PC Magazine, May 16, 1995, pp. 139-156

• Chicago: Under Construction, PC Magazine, April 12, 1994, pp. 183-207