Windows Operating System Case Study

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To explore the principles upon which Windows 7 is designed and the specific components involved in the system

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Windows Operating System Case Study:

Windows Operating System Case Study By Anoop Bhalwal

Case Study Of Windows 7 O.S:

Case Study Of Windows 7 O.S History Design Principles System Components Environmental Subsystems File system Networking Programmer Interface

Objectives:

Objectives To explore the principles upon which Windows 7 is designed and the specific components involved in the system To understand how Windows 7 can run programs designed for other operating systems To provide a detailed explanation of the Windows 7 file system To illustrate the networking protocols supported in Windows 7 To cover the interface available to system and application programmers

Windows 7 :

Windows 7 32-bit/64-bit preemptive multitasking operating system for Intel and AMD microprocessors Key goals for the system: security reliability extensibility portability international support energy efficiency dynamic device support. Supports multiple OS personalities using user-mode subsystems. Windows 7 is for desktops. Windows Server 2008 R2 uses the same internals as 64-bit Windows 7, but with added features for servers.

History:

History In 1988, Microsoft decided to develop a “new technology” (NT) portable operating system that supported both the OS/2 and POSIX APIs. NT supported servers as well as desktop workstations. Originally, NT was supposed to use the OS/2 API as its native environment but during development NT was changed to use the Win32 API, reflecting the popularity of the Windows 3.0 Win16 API. Windows XP was released in 2001 to replace the earlier versions of Windows based on MS/DOS, such as Windows98 and Windows ME. Windows XP was updated in 2005 to provide support AMD64 compatible CPUs, bringing support for 64-bit desktop systems. Windows Vista was released in late 2006, but was poorly received due to initial problems with application and device compatibility and sluggishness on the explosion of low-end “netbook” devices. Windows 7 was released in late 2009, greatly improving on Vista. Windows 8 was released in October 2012 New user interface paradigm (Metro), new type of applications, web store.

Design Principles:

Design Principles Extensibility — layered architecture Kernel layer runs in protected mode and provides access to the CPU by supporting threads, interrupts, and traps. Executive runs in protected mode above the Kernel layer and, provides the basic system services On top of the executive, environmental subsystems operate in user mode providing different OS APIs (as with Mach) Modular structure allows additional environmental subsystems to be added without affecting the executive Portability —Windows 7 can be moved from one hardware platform to another with relatively few changes Written in C and C++ Platform-dependent code is isolated in a dynamic link library (DLL) called the “hardware abstraction layer” (HAL)

Design Principles (Cont.):

Design Principles (Cont.) Reliability —Windows uses hardware protection for virtual memory, and software protection mechanisms for operating system resources Compatibility — applications that follow the IEEE 1003.1 (POSIX) standard can be complied to run on Windows without changing the source code. Applications created for previous versions of Windows run using various virtual machine techniques This is deprecated in Windows 8. Performance —Windows subsystems can communicate with one another via high-performance message passing Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing, scaling to 100s of cores International support — supports different locales via the national language support (NLS) API, use of UNICODE throughout, and providing facilities for differences in date formats, currency, etc.

Windows Architecture:

Windows Architecture Layered system of modules Protected mode — hardware abstraction layer ( HAL ) , kernel, executive. Executive includes file systems, network stack, and device drivers. User mode — collection of subsystems, services, DLLs, and the GUI Environmental subsystems emulate different operating systems Protection subsystems provide security functions Windows services provide facilities for networking, device interfaces, background execution, and extension of the system Rich shared libraries with thousands of APIs are implemented using DLLs to allow code sharing and simplify updates A graphical user interface is built into Win32 and used by most programs that interact directly with the user

Depiction of Windows 7 Architecture:

Depiction of Windows 7 Architecture

System Components — Kernel:

System Components — Kernel Foundation for the executive and the subsystems Never paged out of memory; execution is never preempted Four main responsibilities: thread scheduling interrupt and exception handling low-level processor synchronization recovery after a power failure Kernel is object-oriented, uses two sets of objects dispatcher objects control dispatching and synchronization (events, mutexes, semaphores, threads and timers) control objects (asynchronous procedure calls, interrupts, power notify, process and profile objects)

Kernel — Process and Threads:

Kernel — Process and Threads The process has a virtual memory address space, information (such as a base priority), and an affinity for one or more processors. Threads are the unit of execution scheduled by the kernel’s dispatcher. Each thread has its own state, including a priority, processor affinity, and accounting information. A thread can be one of six states: ready , standby, running, waiting, transition , and terminated.

Kernel — Scheduling:

Kernel — Scheduling Windows scheduler: Pre-emptive (since Windows NT) Multilevel feedback queue The dispatcher uses a 32-level priority scheme to determine the order of thread execution. Priorities are divided into two classes The real-time class contains threads with priorities ranging from 16 to 31 The variable class contains threads having priorities from 0 to 15 Characteristics of Windows 7’s priority strategy Gives very good response times to interactive threads that are using the mouse and windows Enables I/O-bound threads to keep the I/O devices busy Compute-bound threads soak up the spare CPU cycles in the background

Kernel — Scheduling (Cont.) :

Kernel — Scheduling (Cont.) Scheduling can occur when a thread enters the ready or wait state, when a thread terminates, or when an application changes a thread’s priority or processor affinity. Real-time threads are given preferential access to the CPU; but Windows 7 does not guarantee that a real-time thread will start to execute within any particular time limit. This is known as soft real-time.

Executive — Object Manager:

Executive — Object Manager Windows Objects == logical view of resources Kernel objects: not accessible to user code Executive objects: Process, thread, job, file, event, semaphore, mutex, timer, registry key, desktop, symbolic link etc. Windows 7 uses objects for all its services and entities; the object manager supervises the use of all the objects Generates an object handle used by applications to refer to objects Checks security Keeps track of which processes are using each object Objects are manipulated by a standard set of methods, namely create, open, close, delete, query-name, parse and security .

Executive — Naming Objects:

Executive — Naming Objects The Windows executive allows any object to be given a name, which may be either permanent or temporary. Object names are structured like file path names in UNIX. Windows implements a symbolic link object , which is similar to symbolic links in UNIX that allow multiple nicknames or aliases to refer to the same object. A process gets an object handle by creating an object, by opening an existing one, by receiving a duplicated handle from another process, or by inheriting a handle from its parent process. Each object is protected by an access control list. The executive name space is extensible to allow naming of files, registry keys, and other objects with their own special semantics.

Executive — Virtual Memory Manager:

Executive — Virtual Memory Manager The design of the VM manager assumes that the underlying hardware supports virtual to physical mapping, a paging mechanism, transparent cache coherence on multiprocessor systems, and virtual address aliasing. The VM manager in Windows uses a page-based management scheme with a page size of 4 KB for both x86 and AMD64. The VM manager uses a two step process to allocate memory The first step reserves a portion of the process’s address space The second step commits the allocation by assigning space in physical memory or in the paging file on disk

Virtual-Memory Layout (32-bit):

Virtual-Memory Layout (32-bit) Page Directory … 4K page 4K page 4K page 4K page page table 0 … page table 511 … page-directory entry 511 page-directory entry 0 page-table entry 0 page-table entry 511 page-table entry 0 page-table entry 511 Top-level Page Directory Table 0 1 2 3

Virtual Memory Manager (Cont.):

Virtual Memory Manager (Cont.) The virtual address translation in Windows uses several data structures within each process A top-level page directory containing 4 page directory entries (PDEs) of size 8 bytes that may each point to a page directory . Each page directory contains 512 page directory entries, that may each point to a page table . Each page table contains 512 page table entries (PTEs) of size 8 bytes. Each valid PTE points to a 4 KB page frame in physical memory. Invalid PTEs are used by the OS to find pages on disk A 9-bit integer can represent all the values form 0 to 511, therefore, can select any entry in the page directory, or in a page table. This property is used when translating a virtual address pointer to a byte address in physical memory. A physical page can be in one of six states: valid, zeroed, free, standby, modified and bad.

Virtual-to-Physical Address Translation:

Virtual-to-Physical Address Translation Translation for a 32-bit Virtual Address to a Physical Address 2 bit index into top-level page directory to get page directory 9 bit index into page directory to get page directory entry for page table 9 bit index into page table to get page table entry for physical page 12 bits for byte offset within physical page So we have a 3 level hierarchy for Win32 For 64 bit, it is a 4 level hierarchy T O P PDE PTE page offset 31 0

Executive — Process Manager:

Executive — Process Manager Provides services for creating, deleting, and using threads and processes Issues such as parent/child relationships or process hierarchies are left to the particular environmental subsystem that owns the process.

Executive — Local Procedure Call Facility:

Executive — Local Procedure Call Facility This is part of the undocumented Native API, not normally to be used by programs. The ALPC (Advanced Local Procedure Call) component passes requests and results between client and server processes within a single machine. ALPC is used to request operations between the various Windows subsystems and services, as well as to provide the lower layer for standard RPC (Remote Procedure Calls) for a single machine. Standard RPC can connect multiple machines, using TCP/IP or named pipes. When an ALPC channel is created, one of three types of message passing techniques must be specified. First type is used for small messages; the port's message queue provides intermediate storage to copy between processes. Second type avoids copying large messages by pointing to a shared memory section object created for the channel. Third method reads and writes directly into each processes’s address space, and is used by the Win32 GUI.

Executive — I/O Manager:

Executive — I/O Manager The I/O manager is responsible for file systems cache management device and network drivers Keeps track of which installable file systems are loaded, and manages buffers for I/O requests. Works with VM Manager to provide memory-mapped file I/O. Interfaces with the Windows cache manager, which handles caching for the entire I/O system. The cache manager memory-maps files into the kernel memory Also tries to predict the future reading models Flush cache contents for writes: write-back cache, accumulates writes for 4-5 seconds, then writes them

File I/O:

File I/O

Executive — Security Reference Monitor:

Executive — Security Reference Monitor The object-oriented nature of the Windows kernel enables the use of a uniform mechanism to perform runtime access validation and audit checks for every entity in the system. Whenever a process opens a handle to an object, the security reference monitor checks the process’s security token and the object’s access control list to see whether the process has the necessary rights.

Executive – PnP and Power Managers:

Executive – PnP and Power Managers PnP (Plug-and-Play) manager is used to recognize and adapt to changes in the hardware configuration. When new devices are added (for example, PCI or USB), the PnP manager loads the appropriate driver. PnP also keeps track of the resources used by each device. The power m anager controls energy use of by the CPU and devices. Drivers for devices not being used are told to shut off device CPUs are run at lower clock rate and/or lower energy states System can be put into standby mode with only memory on, or Hibernated by writing the contents of memory to disk and turning the system completely off

Environmental Subsystems:

Environmental Subsystems User-mode processes layered over the native Windows executive services to enable Windows to run programs developed for other operating system. Windows 7 uses the Win32 subsystem as the main operating environment; Win32 is used to start all processes. It also provides all the keyboard, mouse and graphical display capabilities. The POSIX subsystem is designed to run POSIX applications following the POSIX.1 standard which is based on the UNIX model.

Environmental Subsystems (Cont.):

Environmental Subsystems (Cont.) Logon and Security Subsystems authenticates users logging on to Windows 7 systems Users are required to have account names and passwords. The authentication package authenticates users whenever they attempt to access an object from a remote system. Windows 7 uses Kerberos as the default authentication package

File System - NTFS:

File System - NTFS Introduced in 1993, to replace older FAT Max file size: 256TB (Win8) Max volume: 256TB (2^64 clusters) Journaling file system The fundamental structure of the Windows 7 file system (NTFS) is a volume Created by the Windows disk administrator utility Based on a logical disk partition May occupy a portions of a disk, an entire disk, or span across several disks All metadata , such as information about the volume, is stored in a regular file NTFS uses clusters as the underlying unit of disk allocation A cluster is a number of disk sectors that is a power of two Because the cluster size is smaller than for the older 16-bit FAT file system, the amount of internal fragmentation is reduced Internally NTFS uses B+ trees to organize file system data Complex data structure, ensures fast lookup

File System — Security:

File System — Security Security of an NTFS volume is derived from the Windows object model. Each file object has a security descriptor attribute stored in the MFT record. This attribute contains the security ID of the owner of the file, and an access control list that states the access privileges that are granted to each user and group that has access to the file.

File System — Compression:

File System — Compression To compress a file, NTFS divides the file’s data into compression units , which are blocks of 16 contiguous clusters. For sparse files, NTFS uses another technique to save space. Clusters that contain all zeros are not actually allocated or stored on disk. Instead, gaps are left in the sequence of virtual cluster numbers stored in the MFT entry for the file. When reading a file, if a gap in the virtual cluster numbers is found, NTFS just zero-fills that portion of the caller’s buffer.

File System — Encryption:

File System — Encryption EFS (Encrypted File System) encrypting individual files or directories of files transparent to the user BitLocker Essentially all of the volume is encrypted There are three levels of key protection Hardware TPM An electronic key plugged into a USB connection User password BitLocker machines should be shutdown rather than placed in standy to avoid attacks on the unencrypted physical memory. Systems protected by BitLocker have a high-degree of security against data theft of lost laptops or stolen systems. Both EFS and Bitlocker only enabled in Professional, Ultimate and Server versions of Windows

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