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Windows NT 3.51 Architecture

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					      CIS 620
 Advanced Operating
      Systems
Lecture 12 – Windows Architecture
       Prof. Timothy Arndt
              BU 331
  Windows NT Design Goals

• Windows NT had a number of design goals:
   Compatibility: Windows 95 interface, support
    for the FAT file system, MS-DOS, OS/2,
    Windows 3.x and POSIX applications and for a
    wide variety of devices and networks.
   Portability: Windows NT runs on both CISC
    and RISC processors.
   Scalability: NT takes full advantage of
    symmetric multiprocessing (SMP) hardware.
    The Microkernel can run on any processor.
Windows NT Design Goals

 Security: Windows NT has a uniform security
  architecture designed to provide a safe
  environment to run mission-critical
  applications. It has met the requirements for C2
  level security.
 Distributed Processing: NT is designed with
  networking built into the base OS. NT supports
  a number of transport protocols and named
  pipes, remote procedure calls (RPCs), and
  Windows Sockets.
Windows NT Design Goals

 Reliability and Robustness: this means that the
  architecture must protect the OS and
  applications from damage. Applications cannot
  read or write outside of their own address
  space. The OS is isolated from applications.
 Localization: NT is offered in many countries
  around the world, in local languages, and
  supports Unicode.
 Extensibility: the modular design of NT allows
  modules to be added to all levels of the OS.
Windows NT Architectural
       Modules
 Windows NT 4.0 architecture is divided into
  two main sections: user mode and kernel
  mode.
   • Kernel mode is a highly privileged mode of
     operation in which the code has direct access to all
     hardware and all memory, including the address
     spaces of all user processes. The part of Windows
     NT running in kernel mode is called the Windows
     NT Executive. It includes the the Hardware
     Abstraction Layer (HAL), Microkernel, and the
     Windows NT Executive Service Modules.
Windows NT Architectural
       Modules
 • User mode is a less privileged processor mode with
   no direct access to hardware. Code running in user
   mode acts directly only in its own address space. It
   uses well-defined operating system application
   program interfaces (APIs) to request system
   services. The environment and integral subsystems
   run in user mode.
 • In Windows NT 4.0, the Windows Manager, the
   Graphics Device Interface (GDI), and graphics
   device drivers have been moved from the Win32
   subsystem to the Windows NT Executive. This was
   done to improve performance, but it reduces the
   reliability of the system.
Windows NT Architectural
       Modules
 Windows NT 4.0 moves further away from a
  pure microkernel architecture.
 Pure microkernel architectures are modular, and
  they keep the number of components running in
  kernel mode to a minimum.
 These architectures are inherently more
  reliable, but they may suffer from performance
  problems due to constant context switches.
 Windows NT is a modified microkernel
  architecture.
Windows NT 3.51
  Architecture
Applications       Logon                    OS/2                      Win32                POSIX
                  Process                   Client                    Client               Client



                                            OS/2                                           POSIX
                                          Subsystem                                       Subsystem


  Protected      Security                                        Win32
Subsystems      Subsystem                                       Subsystem


                                                                                                   User Mode

                                                                                                Kernel Mode


                                                      System Services


                               Security                                         Virtual   I/O Manager
                  Object                      Process         LPC
NT Executive                  Reference                                        Memory
                 Manager                      Manager        Facility                     File Systems
                               Monitor                                         Manager
                                                                                           Cache Manager
                                                                                             Device Drivers
                                                                                               Network Drivers
                                                        Microkernel


                                              Hardware Abstraction Layer (HAL)




                                                        Hardware



               Message Passing
               System T rap
               Hardware Manipulation
Windows NT 4.0 Architecture
      Unix Architecture


                 Application



  X-Windows                         User Mode

                                    Kernel Mode
              System Services
Device Drivers    Process Mgmt., Memory Mgmt, etc.
         Hardware Dependent Code
  Namespace and Object
     Management
 An OS namespace gives applications the ability
  to identify and share resources. The file-system
  namespace is a well known part. Other
  resources include synchronization resources
  and shared memory.
 NT’s Object Manager subsystem implements
  NT’s namespace.
   • The Object Manager is a collection of kernel
     functions that provide uniform resource tracking,
     naming, and security to applications and other
     kernel-mode subsystems.
Namespace and Object
   Management
• Kernel subsystems define Object Manager objects to
  represent the subsystem’s resource types, and rely
  on the Object Manager’s support routines for
  naming and security.
• Processes are represented as process objects, files as
  file objects, etc.
• The Object Manager notifies subsystems that own
  an object when applications close, open, or query
  the object. This is done via method functions
  registered when the object type is defined.
• In response, subsystems can perform actions
  particular to the object type.
   Namespace and Object
      Management
 UNIX’s object-tracking mechanism is not as
  formal as NT’s. It is based on i-nodes.
 Remember that files represent devices and
  sockets as well as “normal” files.
 The kernel notifies file system drivers of
  actions that applications perform on i-nodes.
 This is done by calling functions registered in a
  table that the file system associates with i-
  nodes.
 Both namespaces are hierarchical.
    Process Management

 NT and UNIX are time-sharing OSs that try to
  divide CPU time fairly between applications
  competing for the CPU.
 Neither OS is suitable for a “hard” real-time
  environment.
 NT defines an application using a process
  object, which serves as a container for all
  information about the application.
   • Includes a memory space definition that contains the
     application’s code and data, a table of resources, and
     one or more threads of execution.
    Process Management

 The NT scheduler divides time between threads
  (not between applications). Applications can
  create additional threads, and all of an
  application’s threads share resources and
  memory space.
 The scheduler attempts to give CPU time to the
  highest-priority thread available.
 There are two classes of threads: dynamic (with
  a priority value of between 1 and 15) and real-
  time (with values between 16 and 31).
    Process Management

 Real-time priority values are fixed; the
  scheduler does not adjust those priority values.
   • Typically, only a few OS-owned threads execute in
     the real-time range.
 Same priority threads are scheduled in a
  preemptive, round-robin manner.
 The NT scheduler can also preempt the kernel.
  Further, multiple threads can execute kernel
  code on separate CPUs.
   • These two features allow for SMP.
Process Management
    Process Management

 Process management in modern UNIX systems
  is similar to NT process management.
 UNIX schedulers usually implement three
  priority classes - realtime, system, and dynamic
  - that span priority numbers from 0 to 100.
 The kernels of most UNIX implementations are
  fully preemptible and reentrant.
   • Several varieties of UNIX (HP-UX, AIX, Solaris)
     run on large SMPs with 32 or more CPUs, some run
     on asymmetric multiprocessors. NT is limited to 8
     CPU SMPs.
    Memory Management

 An OSs memory manager is responsible for
  defining virtual address spaces for application
  code and data, and for sharing the physical
  memory resource of the computer among
  applications.
 NT’s Memory Manager defines a 32-bit virtual
  address map for 4GB of virtual memory.
  Usually, NT assigns the low 2GB to the user
  mode and the upper 2GB to the kernel mode.
   • Applications do not have direct access to the kernel-
     mode portion of the address space.
    Memory Management

   • Some versions of NT (e.g., NT Server 4.0,
     Enterprise Edition) support a switch that changes the
     virtual address space division to 3GB for user space
     and 1GB for kernel space.
   • The kernel space permanently maps the NT kernel
     and device drivers, but user-space mapping changes
     to reflect the map of the currently executing thread.
 NT’s Memory Manager implements demand-
  paged virtual memory, in which the Memory
  Manager brings code and data into physical
  memory as an application accesses the code and
  data.
Virtual Memory
   Memory Management

 The Memory Manager implements the features
  of a modern OS
  • Applications can share portions of their address map
    with other applications.
  • Copy-on-write is enabled, for efficient
    implementation of shared memory when changes to
    shared memory need to remain private.
  • Memory mapped files are enabled (changes to the
    mapped file automatically reflect back on the file’s
    on-disk image).
    Memory Management

 NT assigns each application an upper and lower limit
  on physical memory. NT calls this amount of physical
  memory the application’s working set.
 When an application reaches its working set’s upper
  limit and accesses more code or data, the Memory
  Manager uses a least-recently-used algorithm to find
  data in the working set to replace.
 Most UNIX memory managers are similar.
    Memory Management

 Some define an even split between user and
  kernel space and some give the majority to
  applications, leaving only a few hundred MB
  for the kernel.
   • UNIX memory managers differ from NT in that they
     manage memory globally - they do not constrain
     applications to upper and lower limits. The least-
     recently-used algorithm is applied to all
     applications. This can lead to thrashing.
   • Several versions of UNIX support a 64-bit address
     space. Using 64-bit address spaces can help data-
     intensive applications like DB servers.
                Security

 NT’s security capabilities have earned it a C2-
  capable rating (as a standalone non-networked
  system).
 The NT Object Manager’s centralized security
  support means that the Object Manager can
  implement any object - including
  synchronization objects, shared memory, and
  files - with security.
 NT can audit successful and failed attempts to
  access an object.
Security
                Security

 The traditional UNIX security model is much
  less powerful.
 The lack of ACLs and auditing prevent
  traditional UNIX from achieving a C2-capable
  security rating.
   • Major UNIX vendors have implemented proprietary
     versions which implement these features.
   • Some UNIX variants carry a B2 rating which is
     higher than that of NT
   Thin-Client Computing

 UNIX has supported thin-client computing (X-
  terminals) for many years. Until recently,
  however, NT was a single user system.
 Thin-client systems allow a terminal to display
  applications that run on a server somewhere on
  the network.
 Citrix modified NT Server 3.51 to let multiple
  user sessions run on one server: A properly
  equipped server can run as many as 60 to 100
  NT sessions.
   Thin-Client Computing

 Citrix developed the Independent Computing
  Architecture (ICA) protocol to allow clients to
  communicate with the server.
 A portion of the RAM and disk are allocated to
  each session. The OS keeps track of individual
  user’s activity.
 The OS transmits screen output using ICA to
  the client. The client’s keyboard input is
  transmitted to the server.
   • A portion of an application is shared among sessions
     and another portion is not.
   Thin-Client Computing

 In 1997, Microsoft licensed ICA technology
  from Citrix. Microsoft developed another
  protocol called Remote Desktop Protocol
  (RDP).
 RDP runs on Windows CE-based terminals and
  Windows-based PCs.
 ICA supports Windows-based PCs and other
  thin-client devices.
 RDP is supported in Windows NT Server 4.0,
  Terminal Server Edition, released in June 1998.
    Thin-Client Computing

 The Object Manager and Virtual Memory manager
  have been modified to perform in a multi-user
  environment.
   • Every object name created within a session is
     appended with a unique identifier number associated
     with the individual session that created it (SessionID).
 The fact that all processes share the kernel
  address space resulted in kernel resource
  limitations when supporting multiple
  interactive sessions on a single server.
   Thin-Client Computing

 In Windows NT Server 4.0, Terminal Server
  Edition, these limitations were addressed by
  creating a special address range in the kernel,
  called "SessionSpace," that can be mapped on a
  per-session basis.
 A new Windows NT service called “Terminal
  Server” is the controlling process in the
  Terminal Server architecture.
   • It is primarily responsible for session management,
     initiation, and termination of user sessions and
     session event notification.
Session Space
   Thin-Client Computing

 The Terminal Server service is entirely
  protocol-independent, so it can function
  using RDP or a third-party add-on
  protocol such as Citrix’s ICA.
 A user mode protocol extension provides
  assistance to the Terminal Server
  service. It is the responsibility of this
  component to provide protocol-specific
  functions and services, such as licensing,
  session shadowing, client font
  enumeration, and so forth.
Terminal Server Architecture

				
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