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Software Components

PC software components depend upon the Operating System (OS) installed. The OS is a set of programs coordinating all work activity in the PC. Without it and other software, the PC does nothing but heat the room.

The most common PC operating systems are:

  • DOS—The original PC operating system is used to set up and sometimes to troubleshoot PCs. DOS makes the functionality of the computer hardware accessible for running the Windows setup. DOS is the basic set of computer instructions that manages the flow of information in the PC.

  • WindowsThere are several major iterations of Windows: Windows 3.x, Windows 95, Windows 98, and Windows Millennium Edition (Me). Windows Me is the operating system that was delivered with most new PCs. It provides a graphical user interface (GUI) and multitasking capabilities. A version of Windows XP is now filling that role.

  • Windows NTThis is the network server and industrial strength operating system for PCs. It provides a GUI similar to Windows 9x. Windows NT is a more robust operating system than Windows with better isolation of DOS, Windows, and Windows NT applications from each other. This makes for a more reliable operating environment.

  • Windows 2000This is the operating system that has evolved from Windows NT. The user interface is virtually identical to Windows Me. Underneath the user interface is an improved Windows operating system based upon the same core technology used in Windows NT servers. Windows 2000 supports a variety of CPU chips (Windows Me runs only on Pentium and Pentium-compatible CPU chips), runs on two CPUs simultaneously, and supports plug-and-play hardware components. In the long term, variations of Windows 2000, soon to be called Windows XP, will become the Microsoft operating system delivered with all Windows PCs. There are server and workstation versions of Windows 2000. The workstation version of Windows 2000 is called Windows 2000 Professional.

Each operating system has several major components.


DOS is not a knowledge objective of the current A+ certification. However, DOS is still an important operating system because hardware diagnostic and configuration programs use it. For example, a DOS program in most cases performs updating a PC BIOS by flashing NVRAM. Further, DOS commands are implemented in all Windows operating systems.

The PC DOS components are:

  • COMMAND.COM—This is the command interpreter for DOS. Commands typed on the keyboard at the DOS user command prompt are processed first by COMMAND.COM. It interprets the command and initiates the other software activities to complete the command. COMMAND.COM interfaces to MSDOS.COM through the DOS Application Programming Interface (API).

  • MSDOS.SYS—This performs the tasks and schedules the programs required to accomplish the DOS commands. This component is the heart and hands of DOS.

  • IO.SYS—This DOS component handles the detailed I/O activities of DOS. It interfaces to device driver programs that perform the most detailed hardware operation and control activities. IO.SYS works in conjunction with device driver software for standard and special PC hardware.

  • Device drivers—Device drivers tell DOS how to interface properly with an external device and are specified by DEVICE= statements in CONFIG.SYS. Some DOS device drivers are:

    • SMARTDRIVE.EXE—Manages the disk cache and double buffering for SCSI drives

    • HIMEM.SYS—Provides access to extended memory

    • EMM386.EXE—Manages the upper memory area and creates expanded memory from extended memory

  • Application programs—Application program software interfaces with DOS to use the PC hardware to perform useful work for us. It is through the application program that we see the PC. Under DOS, the application program may have its own device drivers and command interpreter functions. Applications are typically a single large program loaded entirely into memory along with the data being worked upon.

DOS as an operating system runs one program at a time. It provides a command-line user interface. See Figure 1–36.

Figure 1-36Figure 1–36 DOS architecture.

Windows Components

Similar to DOS, Windows 3.x knowledge is no longer an A+ certification objective. However, all Windows operating systems have evolved from the first Windows operating system. Windows 3.x best illustrates the evolution from DOS to Windows and helps us understand the further evolution of the current Windows operating systems.

Windows 3.x expanded upon DOS. It used the basic DOS disk and memory driver programs to perform I/O operations. Windows 3.x substitutes a multitasking KRNL.EXE program for MSDOS.SYS and adds to it GUI components, GDI.EXE and USER.EXE. In this manner, Windows 3.x expands on the foundation laid with DOS to provide the cooperative multitasking operating system with a GUI. See Figure 1–37.

Figure 1-37Figure 1–37 Windows 3.x structure.

Windows 3.x components are:

  • Windows Virtual Machine (VM)—The VM program manages the execution of all Windows applications, performing the cooperative multitasking between them.

  • KRNL.EXE—The kernel is the scheduling program managing the DOS and Windows VMs.

  • GDI.EXE—The GDI program is one half of the GUI. It handles the PC display and other graphical devices.

  • USER.EXE—This is the user graphical interface program monitoring user mouse operations and similar tasks. This is the Windows equivalent to the DOS COMMAND.COM.

Windows 95, Windows 98, and Windows Me are referred to here collectively as Windows 9x. Windows 9x components are generally the same as those for Windows 3.x. However, they function differently to provide improved multitasking and GUI features. The improved features are due to Windows 9x incorporating improved file management, networking, Virtual Machine Manager (VMM), and device drivers as shown in Figure 1–38.

Figure 1-38Figure 1–38 General Windows 9x system structure.

The Windows 9x overall architecture has the same basic components. These are:

  • VM—Manages execution of both Windows 16-bit and 32-bit applications. Multitasking has been changed from cooperative multitasking to preemptive multitasking. In preemptive multitasking, no application program can dominate the VM as was possible with cooperative multitasking.

  • API layerConsists of the Windows kernel, the GDI, and user interface programs. The KRNL386.EXE still manages DOS and Windows VMs. The DOS VMs are not shown in Figure 1–38. The GDI.EXE program supports font scaling, color management, printing, and all the drawing on the screen. Finally, the USER.EXE program creates windows and controls mouse movement, dialogue boxes, and message handling and similar functions.

  • Base systemSupports the system VM and the API layer. The base system is responsible for handling key functions implemented in the Windows 9x operating system.

A powerful feature of Windows 9x is its VMs. Under Windows 3.x, VMs used the 80386 CPU virtual 8086 operating mode. In virtual 8086 mode, a VM emulated an 8086 processor with 1 MB of RAM, I/O ports, and CPU registers. Windows 9x VMs are different. A Windows 9x VM contains a memory map of the memory allocated to the application program, identifies any Windows resources allotted to the application, and tracks hardware registers.

Under Windows 9x, there are the system VMs and DOS VMs. The system VM operates in the Pentium CPU's protected mode. All Windows programs, 16 bit or 32 bit, execute from this area and tie themselves into Windows through the API layer. This linkage between the application and the Windows subsystem is made during the program's load sequence. Within this VM, all 16-bit applications share a common address space, common message queue, and memory heaps. This means that, if one 16-bit application crashes, it often corrupts one of these shared areas.

Within this system VM, 16-bit Windows applications are cooperatively multitasked. Windows 32-bit applications do not use shared areas and resources. Each 32-bit application gets its own memory address space from the system VM. The application message queue and the memory heaps reside within its memory address space. The separate memory address spaces for 32-bit applications make them more reliable than 16-bit applications.

DOS VMs are very much like a PC running DOS with 1 MB of memory. The applications running in DOS VMs make service requests of the system, just like they would under DOS, through the Hexadecimal Interrupt 21 (INT21H). These requests, however, are intercepted and passed on to protected-mode code tying into system services. This is similar to the 32-bit file access function of Windows 3.11. Each DOS VM is formed from an invisible VM created during the Windows boot process. This VM contains the global environment for all DOS VMs, including information such as which Terminate-and-Stay-Resident (TSR) programs were loaded from the AUTOEXEC.BAT file. Similar to Windows 3.x, each DOS application runs in its own VM.

The Windows 9x API is designed to maintain backward compatibility to Windows 3.x. Most programs first used with Windows 9x were Windows 3.x applications (Windows 16-bit applications). Windows 9x is a 32-bit operating system, so it also has a 32-bit application API. This 32-bit API is compatible with the Win32 API of Windows NT and Windows 2000. Three main components, the GDI, the user graphical interface program, and the kernel, support the Windows 9x API. All Windows applications use these same modules. The modules additionally have been designed to handle both 32-bit and 16-bit operations.

The Windows 9x base system is comprised of several components including:

  • Device Drivers—Windows 95 uses DOS device drivers as well as newer 32-bit device drivers. Some older DOS applications must have the older drivers loaded in order to function with the PC's hardware components. Windows 95 provides a number of these drivers in the 32-bit form, including those for PCMCIA cards, CD-ROMs, and the mouse.

  • VMM System—This is likely the most important base system component, providing a central core of virtual services. Windows 95 has more than 700 services in the base system, and more than half of those are provided by the VMM. The VMM is a Virtual Device Driver (VxD) that resides in the file DOS386.EXE. It is responsible for handling event coordination (thread supervision), performing interrupt handling, managing memory (virtual and physical), scheduling (time slicing), and controlling the VMs.

  • OS services—This area performs many of the miscellaneous functions of the OS, including fulfilling date and time requests and the plug-and-play configuration system.

  • Network subsystem—The network subsystem is the latest version of the peer-to-peer network, released with Windows for Workgroups. This version uses the file management system as a way to coordinate file access across the network. Because of the Installable File System (IFS) manager, the network subsystem can also provide access to other network systems simultaneously. With the network subsystem comes support for Novell, Transmission Control Protocol/Internet Protocol (TCP/IP), and Server Message Block (SMB).

  • File management system—In Windows 3.1, DOS performed the fixed disk file management functions. This caused most disk operations to be performed in real mode, not in protected mode. Such switching between real and protected mode was inefficient, but necessary, because DOS controlled disk operations. In Windows 9x, Windows controls disk operations with a new file system manager. The IFS manager provides interfaces for local fixed disks, CD-ROMs, and networks.

Windows NT Features and Components

Windows NT components carry the same nomenclature as Windows 3.x and Windows 9x components, but their functionality differs from Windows 9x. From a user perspective, the GUI appears nearly identical, but the VM, API, kernel, and driver programs provide capabilities beyond those of Windows 9x. The Windows NT functionality includes:

  • Large platform support—Windows NT provided support for a wide array of CPU chip platforms, from Intel 386 to Reduced Instruction Set Computer (RISC) processors, like the Million Instructions Per Second (MIPS) R4000 and Digital's Alpha AXP.

  • True 32-bit preemptive multitaskingWindows NT allows preemptive multitasking to DOS, Windows 16-bit, and Windows 32-bit, OS/2, and POSIX applications. The user only learns how to use another application, not another operating system.

  • Easy networkingWindows NT comes with its own networking component built in. It comes with support for TCP/IP networking, Microsoft LAN Manager, Windows for Workgroups, Novell NetWare, Apple Talk, and more.

  • Multiprotocol supportWindows NT supports all key network protocols including TCP/IP, Network Basic Input/Output (NetBIOS) Enhanced User Interface (NETBEUI), and Sequenced Packet Exchange/Internetwork Packet Exchange (SPX/IPX).

  • Large capacitiesWindows NT supports 4 GB of RAM and 16 exabytes of disk space. Disk drive capacity terminology is presented in detail in Chapter 14, but be assured an exabyte is much larger than a gigabyte.

  • Broad file system supportWindows NT provides support for the DOS File Allocation Table (FAT), the OS/2 High Performance File System (HPFS), the Windows NT File System (NTFS), and the CD File System (CDFS).

The same basic architecture as Windows 9x provides these capabilities, including a system VM, an API, and base system functions supporting the system VM and the API. At this point, the similarity ends. Windows NT was written from the ground up to be both a client and a server OS. Its program code is not an improvement on existing Windows programs, but rather all new code designed to be run on high-end PCs and servers. Windows NT provides a user interface similar to Windows 95x.

Windows NT 4.0 lacks the plug-and-play capabilities of Windows 9x and is more limited in the PC hardware that it supports.

Finally, Windows NT was developed without regard for backward compatibility. This means that some Windows 16-bit applications do not run on Windows NT while they run just fine on Windows 9x. In contrast, Windows 9x is not designed to run as network server software. Windows 9x only supports peer-to-peer networking and is not robust enough to operate under heavy networking loads.

Windows NT additional server applications software augments Windows NT by design to customize its functionality to specific networking needs. Microsoft's Structured Query Language (SQL) and Exchange Server programs are examples of such server-centric applications.

Windows 2000

Some Windows 2000 software components have the same names as Windows 9x components. From a user perspective, the interface appears to be the same between Windows Me and Windows 2000. However, underneath, the operating system is very different. The similar Windows NT and Windows 2000 functionality includes:

  • Multiple platform support—Windows 2000 provides support for several CPU chip platforms, from the Intel Pentium Complex Instruction Set Computer (CISC) chips to RISC processors.

  • Robust OSWindows 2000 provides Symmetric Multiprocessing (SMP); 32-bit preemptive multitasking; and the ability to run DOS, Windows 16-bit, Windows 32-bit, and POSIX applications.

  • Network server capabilitiesIt has the ability to act as a network server, providing advanced networking controls, file system security, PC security, and networking security features not found in Windows 9x.

  • Plug-and-play supportWindows 2000 supports a wide variety of multimedia plug-and-play hardware components unlike Windows NT.

  • Reliable operating environmentIt has fewer crashes than Windows 9x. Our experience is that Windows 2000 still vengefully crashes. Such crashes are caused most often by mismatched display driver software, bad network connections, and problematic USB devices. Windows 2000 is very reliable, however, once good drivers are installed.

Windows 2000 provides better memory management, reducing the conflicts between software applications, and provides automatic repair of errant applications. See Figure 1–39. The main reasons for moving to Windows 2000 include:

  • Provides increased reliability and performance over Windows 95 and Windows 98

  • Includes specific features for laptop computers

  • Makes it easier to manage and support with the self-healing application features

  • Includes comprehensive security features

  • Supports USB and IEEE 1394 interfaces

  • Provides an improved user interface over Windows 98

Windows XP is basically the same as Windows 2000 but, with a more modern look and feel.

Study Break: Finding USER.EXE and GDI.EXE

It is time to find key components inside Windows. The two files that we are looking for are USER.EXE and GDI.EXE. These files should be locatable in Windows 95, Windows 98, Windows Me, Windows NT, and Windows 2000 by doing the following:

  1. Click on the Start menu, drag to Find, and finally click on Find Files or Folders.

  2. Type in "User*.*", and then click Find.

  3. Next, type in "GDI*.*", and then click Find.

Both of these files should come right up because they are key components of the Windows operating systems.

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