A system crash: If you're lucky, it only ruins your day. More than likely, you're in for several bad days followed by a few stressful weeks or months. After all, systems rarely fail only once. Rather, they keep crashing until you find the cause and fix the problem.
This primer will show you how to solve problems quickly. Using a tool that costs nothing, you can solve approximately 50% of Windows server and workstation crashes in a few minutes. The tool is WinDbg , the free Windows debugger.
You've probably never used the debugger, don't have it and don't want it. After all, it's a developer's tool, not an administrator's, right? Yes, but what you need to know is remarkably easy to learn, and even a rudimentary familiarity with the debugger could enhance your skills and your resume.
Still hesitant? Think about this: After rebooting a crashed machine, we've brought up the debugger, opened a memory dump file, given the debugger a single command, and learned not only that the cause was a driver, but also the driver's name — all in less than a minute. Granted, the debugger was installed and configured, we knew what commands to use and what to look for.
But so will you by the end of this article.
Why does Windows crash?
To 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.
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.
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 : 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
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
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.
Getting the debugger
The debugger is free and available from Microsoft's Web site. At the site, scroll down until you see the heading, "Installing Debugging Tools for Windows." Select the link, "Install 32-bit version…” and then select the most recent non-beta version and install it. The most recent versions are about 12M-byte downloads. You can do the installation on a PC without restarting it (Don’t be surprised if the site has changed somewhat. Microsoft keeps improving the debugger with releases at least once per year.).
This distribution includes KD.EXE, the command-line kernel debugger; NTSD.EXE, the command-line user-mode debugger; CDB.EXE, the command-line user-mode debugger (a variant of ntsd.exe); and WinDbg, the GUI version of the debugger. WinDbg supports kernel-mode and user-mode debugging, so WinDbg is the one we'll use here.
Setting up the debugger
There are two ways to look at crash data: View what's in memory while the system is stopped (by linking it to a running PC with a null-modem cable, or invoking a product that you pre-installed on the system, such as SoftICE, which lets you step through the code in memory line by line)
Null-modem cables are serial cables that have been configured to send data between two serial ports. They are available at most computer stores. Do not confuse null-modem cables with standard serial cables, which do not connect serial ports.
Given that minimizing interruptions is the goal of most administrators, we opt for the second way: Restart the server or PC, launch the debugger, and open the dump file.
From the program group Debugging Tools for Windows, select WinDbg. After the debugger comes up, you'll immediately notice a lot of … nothing. A blank screen. That's because you have to specify a dump file to analyze and download symbol tables to use in the analysis. Let's take care of the symbol files first.
Symbol tables are a byproduct of compilation. When a program is compiled, the source code is translated from a high-level language into machine code. At the same time, the compiler creates a symbol file with a list of identifiers, their locations in the program, and their attributes. Some identifiers are global and local variables, and function calls. A program doesn't require this information to execute. Therefore, it can be taken out and stored in another file, reducing the size of the final executable.
Smaller executables take up less disk space and load into memory faster than large ones. But there's a flip side: When a program causes a problem, the OS knows only the hex address at which a problem occurred. You need something more than that to determine which program was using that memory space and what it was trying to do. Windows symbol tables hold the answer. Accessing these tables is like laying a map over your system's memory.
Windows symbol files are free from Microsoft's Web site, and the debugger can retrieve them automatically. To set up the debugger to do this, verify that you have a live Internet connection and set the symbol file path in WinDbg by selecting File | Symbol File Path. Then enter the following string:
Substituting your own directory path for c:\local cache. For example, if you want the symbols to be placed in c:\symbols, then set your symbol path to
The location of the symbol table is up to you.
When opening a memory dump, WinDbg will look at the EXE/DLLs and extract version information. It then creates a request to the symbol server at Microsoft, which includes this version information, and locates the precise symbol tables to draw information from. If you have difficulty retrieving symbol files, check that your firewall permits access to http://msdl.microsoft.com.
If you restrict your debugging to memory dumps from the machine you are on, you will need relatively little hard-disk space for the symbol tables. In most cases 5M-bytes will be more than sufficient. But if you plan to look at dumps from other machines that have different Windows versions and patch levels, you'll need more space for the additional symbol files that support those versions.
System update workaround
If you are trying to analyze mini dumps on a machine that had updates installed after the dumps were created (or if you're analyzing a mini dump file from another machine), the drivers found in your system root will be different (newer) than the ones present when the mini dump were created. To solve this, set the executable image file path by selecting File | Image File Path. Then enter the following string: c:\windows\System32; c:\windows\system\System32; http://www.alexander.com/SymServe.
Loading the dump file
To open the dump file that you want to analyze, select File | Open Crash Dump. You'll be asked if you want to save workspace information. Click Yes if you want it to remember where the dump file is. WinDbg looks for the Windows symbol files. WinDbg references the symbol file path, accesses microsoft.com, and displays the results. Close the Disassembly window so you are working in the Command window.
NOTE: Don’t be surprised if the debugger seems rather busy following opening of the dump file, especially the first time you try it. It needs to retrieve symbols and, in the case of mini dumps, it needs to retrieve the binaries. This may take a few minutes. Also, the newer release of WinDbg seems to take longer retrieving driver data as well. Be patient. It is worth the wait!
At this point, WinDbg may return an error message, such as the following one, indicating it could not find the correct symbol file.
*** ERROR: Symbol file could not be found. Defaulted to export symbols for ntoskrnl.exe -
If it does, one of the following three things is usually wrong:
Your path is incorrect; check to make sure there are no typos or other errors in the symbol file path you entered earlier.
Your connection failed; check your Internet connection to make sure it is working properly.
Your firewall blocked access to the symbol files or damaged the symbol file during retrieval.
If your path and connection are solid, then it's likely that the problem is your firewall. If a firewall initially blocks WinDbg from downloading a symbol table, it can result in a corrupted symbol file. Unblocking the firewall and attempting to download the symbol file again does not work; the symbol file remains damaged. The quickest fix is to close WinDbg, delete the symbols folder (which you most likely set at c:\symbols), and unblock the firewall. Now, reopen WinDbg and a dump file. The debugger will recreate the folder and re-download the symbols.
If you see this message, "***** Kernel symbols are WRONG. Please fix symbols to do analysis.", WinDbg was unable to retrieve the proper symbols and it will resort to using the default symbol table. But as the warning suggests, it cannot produce accurate results. Remember that symbol tables are generated when programs are compiled, so there is a symbol table file for every Windows version, patch, hot fix, and so on. Using the wrong symbols to track down the cause of a crash is like trying to steer a ship into Boston Harbor with a chart for San Diego. You must use the right ones, so go back up to the section above and ensure you have the right path set, the connection is good, and it is not blocked.
Look through WinDbg's output. You may see an error message similar to the following that indicates it could not locate the symbols for a third-party driver.
*** ERROR: Module load completed but symbols could not be loaded for driver.dll
Unable to translate address bf9a2700 with prototype PTE
Probably caused by: driver.dll (driver+44bd)
This means that the debugger has found a driver is at fault but, being a third-party driver, there are no symbols for it (Microsoft does not store all of the third-party drivers). You can ignore this. Vendors do not typically ship drivers with symbol files, and they aren't necessary to your work; you can pinpoint the problem driver without them.
With the dump file loaded into WinDbg, it's time to ask for some diagnostic information. While there are loads of commands to use, two are all you need: !analyze –v and , and lmv.
!analyze –v displays information describing the state of a system when it crashed, the fault encountered, and who is the primary suspect.
lmv displays a list of drivers and their path, version and vendor information. It often includes a product description.
If you want to sound like a software engineer, or if at least you don't want to sound clueless, here's how you pronounce the first command: "bang analyze dash vee."
Analysis with !analyze –v
Type !analyze –v on the command line at the bottom of the Command window. The explanation it gives is a combination of English and programmer-speak, but it is nonetheless a great start. In fact, in many cases you may not need to go any further. If you recognize the cause of the crash, you're probably done.
Here's an example. After typing! analyze –v, we receive the following output:
kd> !analyze -v
(This is a very common bugcheck. Usually the exception address pinpoints the driver/function that caused the problem. Always note this address as well as the link date of the driver/image that contains this address.)
Arg1: c0000005, The exception code that was not handled
Arg2: bf9bc4bd, The address that the exception occurred at
Arg3: f69f02bc, Trap Frame
bf9bc4bd 8b4014 mov eax,[eax+0x14]
TRAP_FRAME: f69f02bc -- (.trap fffffffff69f02bc)
ErrCode = 00000000
eax=00000000 ebx=01740000 ecx=010886a0 edx=f69f069c esi=fa07d400 edi=e161f7f8
eip=bf9bc4bd esp=f69f0330 ebp=f69f0344 iopl=0 nv up ei pl nz na pe nc
cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00010202
bf9bc4bd 8b4014 mov eax,[eax+0x14] ds:0023:00000014=????????
LAST_CONTROL_TRANSFER: from bf9ba5cf to bf9bc4bd
f69f0344 bf9ba5cf e161f7f8 e17f8e30 e21e4530 vdriver+0x44bd
f69f06b0 f69f06e0 e2638678 f69f06e4 f69f0890 vdriver+0x25cf
e1bd6b90 1f0507b6 00000000 e1622008 00000010 0xf69f06e0
00000000 00000000 00000000 00000000 00000000 0x1f0507b6
f69f0bf0 805766ef f69f0c78 f69f0c7c f69f0c8c nt!KiCallUserMode+0x4
f69f0c4c bf8733cd 00000002 f69f0c9c 00000018 nt!KeUserModeCallback+0x87
f69f0ccc bf8722a5 bc667998 0000000f 00000000 win32k!SfnDWORD+0xa0
f69f0d0c bf873b38 7196bc2d f69f0d64 00affed0 win32k!xxxDispatchMessage+0x1c0
f69f0d58 805283c1 00afff2c 804d2d30 ffffffff win32k!NtUserDispatchMessage+0x39
f69f0d58 7ffe0304 00afff2c 804d2d30 ffffffff nt!KiSystemService+0xc4
00afff08 00000000 00000000 00000000 00000000 SharedUserData!SystemCallStub+0x4
bf9bc4bd 8b4014 mov eax,[eax+0x14]
Look for a section labeled "Debugging Details." Then, scan down until you find DEFAULT_BUCKET_ID:. This provides the general category of the failure. It shows DRIVER_FAULT, indicating that a driver is the likely culprit. Scanning further down to IMAGE_NAME, we see vdriver.dll. We have a suspect!
Analysis with lmv
The next step is to confirm the suspect's existence and find any details about him. Typing lm in the command line displays the loaded modules; v instructs the debugger to output in verbose (detail) mode, showing all known details for the modules. This is a lot of information. Locating the driver of interest can take a while, so simplify the process by selecting edit | Find.
Here's an example of output generated by the lmv command:
bf9b8000 bfa0dc00 VDriver (no symbolic information)
Loaded symbol image file: VDriver.dll
Image path: \SystemRoot\System32\VDriver.dll
Checksum: 00058BD5 Timestamp: Fri Sep 28 10:12:47 2001 (3BB4855F)
File version: 126.96.36.1996
Product version: 188.8.131.526
File flags: 8 (Mask 3F) Private
File OS: 40004 NT Win32
File type: 3.4 Driver
File date: 00000000.00000000
CompanyName: Video Technologies Inc.
ProductName: VDisplay Driver for Windows XP
FileDescription: Video Display Driver
LegalCopyright: Copyright© Video Technologies Inc. 2000-2004
Support: (800) 555-1212
Use File | Find to locate the suspect driver. If the vendor was thorough, complete driver/vendor detail is revealed
The amount of information you see depends upon the driver vendor. Some vendors put little information in their files; others, such as Veritas, put in everything from the company name to a support telephone number! If a vendor is thorough, the results from the command will be similar to those shown here.
After you find the vendor's name, go to its Web site and check for updates, knowledge base articles, and other supporting information. If such items don't exist or resolve the problem, contact them. They may ask you to send along the debugging information (it is easy to copy the output from the debugger into an e-mail message or Word document), or they may ask you to send them the memory dump (zip it up first, both to compress it and protect data integrity).
Not aways easy
Finding out what went wrong is often a simple process, but it isn't always so. At least 50% of the time (often 70%), the debugger makes the reason for a crash obvious. But sometimes the information it provides is misleading or insufficient. What do you do then?
If you have recurring crashes but no clear or consistent reason, it may be a memory problem. Download the free test tool, Memtest86. This simple diagnostic tool is quick and works great.
Many people discount the possibility of a memory problem, because they account for such a small percentage of system crashes. However, they are often the cause that keeps you guessing the longest.
The operating system is the culprit
Not likely! As surprising as it may seem, the operating system is rarely at fault. If ntoskrnl.exe (Windows core) or win32.sys (the driver that is most responsible for the "GUI" layer on Windows) is named as the culprit, and they often are, don't be too quick to accept it. It is far more likely that some errant third-party device driver called upon a Windows component to perform an operation and passed a bad instruction, such as telling it to write to non-existent memory. So, while the operating system certainly can err, exhaust all other possibilities before you call Microsoft! The same goes for debugging Unix, Linux, and NetWare.
Wrong driver named
Often you will see an antivirus driver named as the cause. For instance, after using !analyze –v, the debugger reports a driver for your antivirus program at the line "IMAGE_NAME". This may well be the case, but bear in mind that such a driver can be named more often than it is guilty. Here's why: For antivirus code to work it must watch all file openings and closings. To accomplish this, the code sits at a low layer in the operating system and is constantly working. In fact, it is so busy it will often be on the stack of function calls that was active when the crash occurred, even if it did not cause it. Because any third-party driver on that stack immediately becomes suspect, it will often get named. From a mathematical standpoint it is easy to see how it will so often be on the stack whether it actually caused a problem or not.
Little or no vendor information
Not all vendors include needed information (not even their name!). If you use the lmv command and turn up nothing, look at the subdirectories on the image path (if there is one). Often one of them will be the vendor name or a contraction of it. Another option is to search Google. Type in the driver name and/or folder name. You'll probably find the vendor as well as others who have posted information regarding the driver.
When systems crash your first objective is to get them up and running. Your second is to fix the problem to prevent future crashes. Be willing to use any tool that can help you — even the Windows debugger. It won't give you the cause of every crash event, but it can help you solve 50% or more with two simple commands.
Smith is president and founder of Alexander LAN, Inc. He can be reached at firstname.lastname@example.org