Windows Operating System Versions

This book covers the three most recent versions of the Microsoft Windows operating system based on the Windows NT code base: Windows 2000, Windows XP (32-bit and 64-bit versions), and Windows Server 2003 (32-bit and 64-bit versions). Unless specifically stated, the text applies to all three versions. As background information, Table 1-1 lists the releases of the Windows NT code base, their internal version number, and the external product name.

Table 1-1. Windows Operating System Releases

Product Name	Internal Version Number	Release Date
Windows NT 3.1	3.1	July 1993
Windows NT 3.5	3.5	September 1994
Windows NT 3.51	3.51	May 1995
Windows NT 4.0	4.0	July 1996
Windows 2000	5.0	December 1999
Windows XP	5.1	August 2001
Windows Server 2003	5.2	March 2003

Windows NT vs. Windows 95

From the initial announcement of Windows NT, Microsoft made it clear that it was to be the long-term replacement for Windows 95 (and its subsequent releases, Windows 98 and Windows Millennium Edition). The following list highlights some architectural differences and advantages that Windows NT (and its subsequent releases) has over Windows 95 (and its subsequent releases):

• Windows NT supports multiprocessor systems Windows 95 doesn't.

• The Windows NT file system supports security (such as discretionary access control). The Windows 95 file system doesn't.

• Windows NT is fully a 32-bit (and now 64-bit) operating system it contains no 16-bit code, other than support code for running 16-bit Windows applications. Windows 95 contains a large amount of old 16-bit code from its predecessors, Windows 3.1 and MS-DOS.

• Windows NT is fully reentrant significant parts of Windows 95 are nonreentrant (mainly the 16-bit code taken from Windows 3.1). This nonreentrant code includes the majority of the graphics and window management functions (GDI and USER). When a 32-bit application on Windows 95 attempts to call a system service implemented in nonreentrant 16-bit code, the application must first obtain a system-wide lock (or mutex) to block other threads from entering the nonreentrant code base. And even worse, a 16-bit application holds this lock while running. As a result, although the core of Windows 95 contains a preemptive 32-bit multithreaded scheduler, applications often run single threaded because so much of the system is still implemented in nonreentrant code.

• Windows NT provides an option to run 16-bit Windows applications in their own address space Windows 95 always runs 16-bit Windows applications in a shared address space, in which they can corrupt (and hang) each other.

• Process shared memory on Windows NT is visible only to the processes that are mapping the same shared memory section. On Windows 95, all shared memory is visible and writable from all processes. Thus, any process can write to and corrupt shared memory being used by other cooperating processes.

• Windows 95 has some critical operating system pages that are writable from user mode, thus allowing a user application to corrupt or crash the system.

The one thing Windows 95 can do that Windows NT based systems will never do is run all older MS-DOS and Windows 3.1 applications (notably applications that require direct hardware access) as well as 16-bit MS-DOS device drivers. Whereas 100 percent compatibility with MS-DOS and Windows 3.1 was a mandatory goal for Windows 95, the original goal for Windows NT was to run most existing 16-bit applications while preserving the integrity and reliability of the system.