Appendix B: Tools for Third Parties

Kernel-Mode Code Development

Some kernel-mode code errors show up immediately during testing. But other types of errors can take a long time to cause a crash, making it difficult to determine where the problem originates. In addition, it is not easy for driver developers to fully test kernel-mode code because it is difficult to simulate all the workload, hardware, and software variables that might be encountered in a production environment.

To address these issues, Windows 2000 Server includes the following features and tools to help developers produce better drivers:

• 
  Pool Tagging
  
• 
  Driver Verifier
  
• 
  Device Path Exerciser
  

Pool Tagging

One common error is to let a kernel-mode component write outside of its memory allocation. This action can corrupt the memory of another kernel-mode component and cause a system failure.

Another common mistake is to allocate memory for a driver process and then fail to release it when the process is finished, creating a memory leak. Memory leaks slowly consume more and more memory and eventually exhaust the shared memory pool, which causes the system to fail. This scenario may take a long time to develop. For example, a driver that requests a small amount of memory and only forgets to release that memory in rare situations will take a long time to exhaust the memory pool.

Both types of errors can be hard to track down. To help developers find and fix such memory problems, Pool Tagging (also known as the Special Pool), has been added to Windows 2000. For testing purposes, Pool Tagging lets kernel-mode device driver developers make all memory allocations to selected device drivers out of a special pool, rather than a shared system pool. The end of the special pool is marked by a Guard Page. If an application tries to write beyond the boundary of their memory allocation, it hits a Guard Page, which causes a system failure. Once alerted by the system failure, a developer can track down the cause of the memory allocation problem.

To help developers find memory leaks, Pool Tagging also lets developers put an extra tag on all allocations made from the shared pool to track tasks that make changes to memory.

Driver Verifier

The Driver Verifier tests for specific sets of error conditions. Once an error condition is found, it is added to the existing suite of tests for future testing purposes. The Driver Verifier can test for the following types of problems:

• 
  Memory corruption. The Driver Verifier checks extensively for common sources of memory corruption, including using un-initialized variables, double releases of spinlocks, and pool corruption.
  
• 
  Writing to pageable data. This test looks for drivers that access pageable resources at an inappropriate time. The problems that result from these types of errors can result in a fatal system error, but may only appear when a system is handling a full production workload.
  
• 
  Handling memory allocation errors. A common programming error is neglecting to include adequate code in the driver to handle a situation when the kernel cannot allocate the memory the driver requests. The Driver Verifier can be configured to inject random memory allocation failures to the specified driver, which allows developers to quickly determine how their drivers will react in this type of adverse situation.
  

Device Path Exerciser

Devctl, the Device Path Exerciser, ships in the Hardware Compatibility Test 8.0 test suite, available at http://www.microsoft.com/hwtest/TestKits/.

Driver Signing

Windows 2000 includes a new feature called Driver Signing. Driver Signing is included in Windows to help promote driver quality by allowing Windows 2000 to notify users whether or not a driver they are installing has passed the Microsoft certification process. Driver Signing attaches an encrypted digital signature to a code file that has passed the Windows Hardware Quality Labs (WHQL) tests.

Microsoft will digitally sign drivers as part of WHQL testing if the driver runs on Windows 98 and Windows 2000 operating systems. The digital signature will be associated with individual driver packages and will be recognized by Windows 2000. This certification proves to users that the drivers they employ are identical to those Microsoft has tested, and notifies users if a driver file has been changed after the driver was put on the Hardware Compatibility List.

If a driver being installed has not been digitally signed, there are three possible responses:

• 
  Warn: lets the user know if a driver that’s being installed hasn’t been signed and gives the user a chance to say “no” to the install. Warn will also give the user the option to install unsigned versions of a protected driver file.
  
• 
  Block: prevents all unsigned drivers from being installed.
  
• 
  Ignore: allows all files to be installed, whether they’ve been signed or not.
  

• 
  Vendors wishing to have drivers tested and signed can find information on driver signing at http://www.microsoft.com/hwtest/. Only signed drivers are published on the Windows Update Web site at http://windowsupdate.microsoft.com/default.htm.
  

Developing and debugging user-mode code

User mode is the portion of the operating system in which application software runs. Windows 2000 includes a new tool, PageHeap, which can help developers find memory access errors when they are working on non-kernel-mode software code.

Heap refers to the memory used to temporarily store code. Heap corruption is a common problem in application development. Heap corruption typically occurs when an application allocates a block of heap memory of a given size and then writes to memory addresses beyond the requested size of the heap block. Another common cause of heap corruption is writing to a block of memory that has already been freed. In both cases, the result can be that two applications try to use the same area of memory, leading to a system failure. To help developers find coding errors in memory buffer use faster and more reliably, the PageHeap feature has been built into the Windows 2000 heap manager.

When the PageHeap feature is enabled for an application, all heap allocations in that application are placed in memory so that the end of the heap allocation is aligned with the end of a virtual page of memory. This arrangement is similar to the tagged pool described for kernel memory. Any memory reads or writes beyond the end of the heap allocation will cause an immediate access violation in the application, which can then be caught within a debugger to show the developer the exact line of code that is causing heap corruption.

Appendix C: Windows 2000 OS and Memory Protection

At the center of the reliability and availability improvements in Windows 2000 are new protections for the operating system and memory. Many of the problems that cause instability can be traced to unwanted affects on the core of the operating system, the kernel, where essential system services are performed. Because it controls the entire operating system, code errors that affect the kernel have a major impact on reliability. Errors that affect memory are also a common source of instability.

Windows 2000 improves reliability by providing early detection and prevention of improper memory management practices in applications, kernel components, and device drivers. The operating system is designed to gracefully manage application and system errors and exceptions, without bringing down the server. In addition, to ensure that one program’s fault will not affect the operating system or other programs, protected subsystems isolate programs in unique memory locations.

T
o make it easier to visualize the specific improvements in Windows 2000 that address instability issues, this appendix provides an overview of the operating system architecture and memory management.

Figure 1: The Windows 2000 Server Architecture is made up of user-mode and kernel-mode components. User mode is the portion of the operating system in which application software runs. Kernel mode is the portion that interacts with computer hardware. Many of the operating system reliability improvements add protection for the kernel-mode processes.

The Windows 2000 operating system provides the environment in which applications run. To do this, it contains a collection of small, self-contained software components that work together to perform tasks. Each component provides a set of functions that act as an interface to the rest of the system. This collection of modules provides the means to access processor and all other hardware resources. The operating system also provides a mechanism by which applications and components may communicate with one another.

Kernel Mode vs. User Mode

Windows 2000 divides the executing code into the following two areas or modes.

User Mode

Software in user mode operates in a non-privileged state with limited access to system resources. For example, this software can’t directly access hardware. Windows 2000-based applications and protected subsystems run in user mode. The protected subsystems run in their own protected space and do not interfere with each other. They are divided into the following two groups:

• 
  Environment subsystems (see upper-right area of Figure 1 above) are services that provide application programming interfaces (APIs) specific to an operating system. Using the environment subsystems, Windows 2000 is able to run applications written for different operating systems, such as OS/2, using these APIs.
  
• 
  Integral subsystems are services that provide interfaces with important operating system functions such as security and network services. The four boxes in the upper-left area of Figure 1 above represent the integral subsystems.
  

Kernel Mode

In kernel mode, software can access all the system resources such as computer hardware and sensitive system data. The kernel-mode software constitutes the core of the operating system and can be grouped as follows:

• 
  Executive contains system components that are responsible for providing system services to environment subsystems and other executive components. They perform system tasks such as input/output (I/O), file management, virtual memory management, resource management, and interprocess communications.
  
• 
  Device drivers translate calls from components, such as a request to print, into hardware manipulation.
  
• 
  Hardware abstraction layer (HAL) isolates the rest of the Windows 2000 Executive from the specific hardware, making the operating system compatible with multiple processor platforms.
  
• 
  Microkernel manages the microprocessor. It performs crucial functions such as scheduling, interrupt, exception dispatching, and multiprocessor synchronization.
  

Memory Model

Windows 2000 adds features to address some of the potential challenges that arise as different processes share memory. To understand these improvements, it helps to understand the basics of how Windows 2000 manages memory.

W
indows 2000 uses a Virtual Memory Manager (VMM) to manage the use of virtual and physical memory. (Shown as the Memory Manager in Figure 1, above.)

Figure 2:To allow a number of applications to share a finite amount of RAM, the Virtual Memory Manager swaps pages of memory between virtual memory and hard disk space.

Virtual memory refers to how the operating system makes memory available to applications. Windows 2000 supports 4 gigabytes (GB) of virtual memory. The upper 2 GB is reserved for kernel-mode processes and the lower 2 GB is shared by kernel-mode and user-mode processes.

Physical memory refers to the RAM chips installed in the computer. VMM uses a memory-mapping table to keep track of the virtual addresses that belong to each process and where the actual data referenced by these addresses resides in physical memory. To let a number of applications share memory so they can run at once, the VMM uses a process called paging to swap memory contents between RAM and disk storage. The contents being swapped are called pagefiles.

For More Information

For the latest information on Windows 2000, check out our Web site at http://www.microsoft.com/windows2000 and the Windows 2000/NT Forum at http://computingcentral.msn.com/topics/windowsnt.

See also:

Windows 2000 Server and Advanced Server home page: http://www.microsoft.com/windows2000/server

Windows 2000 Datacenter Server home page: http://www.microsoft.com/windows2000/datacenter

Microsoft Application Center 2000 home page: http://www.microsoft.com/applicationcenter/

Introduction to reliability and availability in Windows 2000 Server: http://www.microsoft.com/windows2000/guide/server/overview/reliable/default.asp

Microsoft Press Windows 2000 resources: http://mspress.microsoft.com/Windows2000

Microsoft Support options page: http://support.microsoft.com/directory/overview.asp.

Microsoft Certified Support Centers home page: http://www.microsoft.com/support/mcsc/

Microsoft Enterprise Services home page:

http://www.microsoft.com/msf

Microsoft Operations Framework white papers: http://www.microsoft.com/trainingandservices/MOFoverview

Microsoft Certification home page: http://www.microsoft.com/trainingandservices/default.asp

Hardware Design Guide Version 2.0 for Microsoft Windows NT Server: http://msdn.microsoft.com/library/books/serverdg/hardwaredesignguideversion20formicrosoftwindowsntserver.htm

Server Design FAQ: http://www.microsoft.com/HWDEV/xpapers/SDG2FAQ/FAQ1.htm

Microsoft Windows Hardware Quality Labs home page: http://www.microsoft.com/hwtest/default.asp

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