Windows crashs? What is the Reason Behind?- Part1

Till date, Windows has been used most commonly on the x86 processor. The x86 implements a protection mechanism that lets multiple programs run simultaneously without stepping on each other's toes. This protection comes in four levels of privilege or access to system memory and hardware. Two of these levels are commonly referred to as kernel mode and user mode.

Kernel mode is the most privileged state of the x86. Both the Windows OS and drivers are considered trusted, and, therefore, run in kernel mode. This ensures unfettered access to system resources and the ability to maximize performance. Other software is assigned to user mode, the least-privileged state of the x86, restricting direct access to much of the system. Applications, such as Microsoft Word, run in user mode to guard against applications corrupting system-level software and each other.
• Although kernel-mode software is protected from applications running in user mode, it is not protected from other kernel-mode software. For example, if a driver erroneously accesses a portion of memory that is being used by other software (or not specifically marked as accessible to drivers), Windows stops the entire system. This is called a bug check or a crash, and Windows displays the popularly known Blue Screen of Death (BSOD). About 95% of Windows system crashes are caused by buggy software (or buggy device drivers), almost all of which come from third-party vendors. The remaining 5% is due to malfunctioning hardware devices, which often prompt crashes by corrupting memory contents.

• Microsoft’s analysis of crash root causes indicates:
-70% caused by third-party driver code
-15% caused by unknown (memory is too corrupted to tell)
-10% caused by hardware issues
-5% caused by Microsoft code

• There are lots of third-party drivers! From online crash analysis database:
55,000 unique drivers – 24 new/day (28,000 in 2004)
220,000 total drivers – 98 revised/day (130,000 in 2004)
Many Devices
Over 1,263,300 distinct Plug and Play (PnP) IDs (680,000 in 2004)
1,600 PnP IDs added every day

Another little-known fact is that most crashes are repeat crashes. Few administrators can resolve system crashes immediately. As a result, they typically happen again and again. It's common to see weeks and months pass before the answer is found. By solving a crash immediately after the first occurrence, you can prevent time-consuming and costly repeat crashes.
We'll focus on solving crashes under Windows 2000, XP and Server 2003. The process is identical for Windows servers and desktops. With respect to the debugging and interpretation process, this information applies with remarkably little differences to other operating systems, such as Linux, Unix and NetWare.
Getting started
To resolve system crashes using WinDbg, you need the following:
• A PC with 25M bytes of hard-disk space, a live Internet connection and Microsoft Internet Explorer 5.0 or later.
• A PC running Windows Server 2003, Windows 2000 or Windows XP.
• The latest version of WinDbg .
• A memory dump (the page file must be on C: for Windows to save the memory dump file).

The memory dump is a snapshot of what the system had in memory when it crashed. Few things are more cryptic than a dump file at first glance. Yet it is the best place to go for information on a crash. You can try to get this data in other ways - a user or administrator may remember what the system was doing when it crashed, or that they installed a new hardware device recently, in which case you can check related drivers or hardware - but they could also forget, providing incomplete or inaccurate information.

Windows Server 2003, 2000 and XP create three types of memory dump files:

-Small or mini dump :
-Kernel dump :
-Complete or full dump:

Small or mini dump : A mini dump is a tiny 64K-byte file. One reason it's so small is that it doesn't contain any of the binary or executable files that are in memory at the time of a system crash. The .exes are needed for full and proper crash analysis, therefore, mini dumps are of limited value without them. However, if you are debugging on the machine that created the dump file, the debugger can find them in the System Root folders, unless they were changed by a system update (we'll provide a workaround for this later). XP and Server 2003 produce mini dumps by default, one for each crash event, as well as a full dump file. While it saves all mini dumps, the system only saves the most recent full dump. Windows 2000 can save mini dumps, but by default it is set to save only a full dump.

Kernel dump : This is equal to the amount of RAM occupied by the operating system's kernel. For an XP PC with 512M bytes of RAM, this is usually around 60M bytes, but it can vary. For most purposes, this crash dump is the most useful. It is significantly smaller than the full memory dump, but it only omits those portions of memory that are unlikely to have been involved in the crash.

Complete or full dump : This is equal to the amount of RAM in the box. Therefore, a machine with 512M bytes of RAM creates a 512M-byte dump file (plus a little). While a full dump contains all possible data and executables the memory has to offer, its sheer size can make it awkward to save or transfer to another machine for debugging. Windows 2000 produces a full dump by default.

Because XP and 2003 are set up to save a mini dump for every crash event, there should be a mini dump file for every crash the machine has had since it was turned on. This data can be extremely valuable, giving you a rich history to inspect.

Saving a memory dump
To resolve system crashes through the inspection of memory dumps, set your servers and PCs to automatically save them with these steps:

Right-click on My Computer
Select Properties
Select Advanced
In the Start up and Recovery section, select Settings; this displays the Startup and Recovery dialog box
In the Write debugging information section, select kernel memory dump
While still in the Start up and Recovery dialog box, ensure that the following options are checked in the System failure section:

Write an event to the system log
Send an administrative alert
Automatically restart
In the Write debugging information, you have the option to save only the most recent dump file or to have the system rename the existing dump file before it creates a new one. We prefer saving the dump files because previous dump files may provide additional or different information - however, space can be an issue, so set this option according to your needs.

The Write debugging information section also tells you where the dump file will be created. On XP and 2003 systems, mini dumps are located at %SystemRoot%\Minidump, or c:\Windows\Minidump; kernel and full dumps are located at %SystemRoot%\MEMORY.DMP or c:\Windows\MEMORY.DMP. For Windows 2000, memory dump files are located at c:\winnt\memory.dmp.

If you don't have a dump file on your machine, you can get one from another system or download one here. This kernel dump is about 20M bytes zipped and 60M bytes extracted. It was created using a testing tool that generates a system crash.


(More on debugging side, Check out Part-2)