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Windows 9x
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===Kernel=== {{Main|Architecture of Windows 9x}} Windows 9x is a series of [[Monolithic application|monolithic]] 16/32-bit operating systems. Like most operating systems, Windows 9x consists of [[kernel (operating system)|kernel space]] and [[Userland (computing)|user space]] memory. Although Windows 9x features some [[memory protection]], it does not protect the first megabyte of memory from [[Userland (computing)|userland]] applications for compatibility reasons. This area of memory contains code critical to the functioning of the operating system, and by writing into this area of memory an application can [[Crash (computing)|crash]] or [[Hang (computing)|freeze]] the operating system. This was a source of instability as faulty applications could accidentally write into this region, potentially corrupting important operating system memory, which usually resulted in some form of system error and halt.<ref>{{Cite web|url=http://www.microsoft.com/windowsxp/expertzone/chats/transcripts/bottchat.mspx|title=Transcript: Chat with Ed Bott and Carl Siechert, Co-Authors of Microsoft Windows XP Inside Out|date=November 21, 2001|website=microsoft.com|publisher=[[Microsoft]]|archive-url=https://web.archive.org/web/20040918191218/http://www.microsoft.com/windowsxp/expertzone/chats/transcripts/bottchat.mspx|archive-date=September 18, 2004|access-date=April 20, 2019}}</ref> ====User mode==== The user-mode parts of Windows 9x consist of three subsystems: the Win16 subsystem, the Win32 subsystem and MS-DOS.<ref>{{Cite web|url=http://www.microsoft.com/technet/archive/win95/rk31_arc.mspx?mfr=true|title=Windows 95 Architecture Components|website=[[Microsoft Technet]]|publisher=[[Microsoft]]|archive-url=https://web.archive.org/web/20080207122429/http://www.microsoft.com/technet/archive/win95/rk31_arc.mspx?mfr=true|archive-date=February 7, 2008|access-date=April 20, 2019}}</ref> Windows 9x/Me set aside two blocks of 64 [[KiB]] memory regions for [[Graphics Device Interface|GDI]] and heap resources. By running multiple applications, applications with numerous GDI elements or by running applications over a long span of time, it could exhaust these memory areas. If free system resources dropped below 10%, Windows would become unstable and likely crash.<ref>{{Cite web|url=http://www.apptools.com/rants/resources.php|title=Resource Management Under Microsoft Windows|last=White|first=Gary|website=www.apptools.com|access-date=April 20, 2019}}</ref> ====Kernel mode==== The kernel mode parts consist of the Virtual Machine Manager (VMM), the Installable File System Manager ([[IFSHLP.SYS|IFSHLP]]), the Configuration Manager, and in Windows 98 and later, the [[Windows Driver Model|WDM]] Driver Manager (NTKERN).<ref>{{Cite web|url=http://www.microsoft.com/technet/archive/win98/reskit/part6/wrkc28.mspx?mfr=true|title=Chapter 28 - Windows 98 Architecture|website=[[Microsoft Technet]]|publisher=[[Microsoft]]|archive-url=https://web.archive.org/web/20071221222004/http://www.microsoft.com/technet/archive/win98/reskit/part6/wrkc28.mspx?mfr=true|archive-date=December 21, 2007|access-date=April 20, 2019}}</ref> As a 32-bit operating system, virtual memory space is 4 [[GiB]], divided into a lower 2 GiB for applications and an upper 2 GiB for kernel per process.
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