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Article ID: 72379 - Last Review: January 5, 2015 - Revision: 1.1

This article was previously published under Q72379
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The WDEB386 debugger provided in the Windows Software Development Kit (SDK) has many invaluable features; however, it also has a number of shortcomings. This article describes some of the things that WDEB386 can and cannot do, and provides a few usage tips.


Reasons for Using WDEB386

The WDEB386 debugger was originally written as a Microsoft internal tool for developing and debugging the enhanced mode layer of Windows. As such, it retains a number of the advanced features that are necessary to debug a multitasking, protected mode system. At the same time, the low-level nature of this debugging environment can be unwieldy and confusing in many situations. However, there are many situations where this debugger is particularly useful, or even totally necessary, to diagnose problems such as the following:

  • Tracing through low-level code that CVW will not trace
  • Viewing virtual/linear/physical memory
  • Viewing advanced 386 processor data, such as the GDT, LDT, IDT, and all of the PMODE registers
  • Tracing hardware interrupt handlers
  • Tracing terminate-and-stay-resident (TSR) programs, or MS-DOS device drivers
  • Displaying the status of virtual machines (VMs)
  • Monitoring all interrupts and exceptions in enhanced mode
  • Developing and debugging virtual devices (VxDs) for enhanced mode
This is not an exhaustive list; however, it should serve to illustrate some of the situations where the WDEB386 debugger might typically be used.

Breaking into the Debugger on Startup

One command-line option that was not mentioned in Chapter 9 of the "Microsoft Windows Software Development Kit Tools" manual is the /B option. Specifying /B on the WDEB386 command line instructs the debugger to halt execution during Windows startup. This option does not guarantee that the debugger will halt execution on the very first instruction run. In fact, the debugger does not halt execution until after Windows has loaded VxDs, just prior to initialization.

Breaking into the Debugger in General

When WDEB386 is running, execution of the current instruction stream can be halted with the CTRL+ALT+SYS RQ key combination. This will not stop execution at the precise location of the keyboard interrupt; execution will halt at a location in the virtual machine manager (VMM). The register contents of the interrupted virtual machine can be inspected using the .VM command (see below).

Alternatively, breakpoints can be set with the BP command, or with interrupt instructions assembled directly into the code. Either an INT 1 or INT 3 instruction can be used. The difference is that an INT 1 will produce an "Unexpected trace interrupt" message and stop on the instruction AFTER the INT 1. This message does not indicate an error condition and can be ignored. An INT 3 will break directly on the INT and not produce the message. Once a breakpoint instruction is hit, it can be removed permanently with the "Z" command. This command replaces the INT machine language with NOPs (No OPeration).

Also, if the necessary hardware is available, the nonmaskable interrupt (NMI) can be used to break into the debugger. This usually means having an external "STOP" button connected to a debugging card installed in a slot of the development machine. Some machines may have the capability of connecting a front panel button to the NMI line on the machine bus. In any case, using NMI has the advantage of being able to break into a machine that has "hung" with interrupts disabled.

For programmers developing virtual device drivers (VxDs), the Debug_Out macro is available to combine sending an ASCII string to the debug terminal and executing an INT 1, which will break to the debugger.

Using WDEB386 in Standard Mode

The WDEB386 debugger is provided mainly for enhanced mode debugging; however, it can also be used in standard mode on a 386 processor. In general, operation of the WDEB386 debugger in standard mode is the same as in enhanced mode, except that a number of features are unavailable, particularly in Windows 3.0.

For example, the "/b" option to break on startup is only available in enhanced mode on Windows 3.0. It is available in standard mode on Windows 3.1. Many of the "dot" commands (commands prefixed with a period) are provided for enhanced mode and are not available in standard mode.

Determining the State of the Processor

Once control has been given to the debugger, the prompt character used will provide the protected mode status of the processor. The following list shows what prompt characters may be displayed and the meaning of each:
    Character  Meaning
    ---------  -------

       >       The processor is in real mode
       #       The processor is in protected mode
       -       The processor is in virtual 8086 mode
The mode the processor is in will be a good indication of what code is being executed. For example, if the prompt is a "-" (hyphen), the current instruction stream is somewhere in MS-DOS, the BIOS, or possibly in a TSR or MS-DOS device driver. This is because the enhanced mode layer of Windows must switch the processor to V86 mode to run MS-DOS or BIOS functions. Alternatively, if the prompt is a "#" (number sign), protected mode code -- which could be a Windows-based application, DLL, or even the enhanced mode layer -- is running.

One of the most important aspects of "knowing what is running" when using WDEB386 under Windows in enhanced mode is some awareness of WIN386.EXE. This module consists of the VMM (virtual machine manager) and all VxDs (virtual devices). These components are often collectively referred to as the "enhanced mode layer," "ring zero code," or just "WIN386." Under Windows 3.0 and 3.1, and Windows for Workgroups, versions 3.0, 3.1, and 3.11, if the debugger prompt is a "#" and the value of the CS register is 0028h, it means the machine is stopped in WIN386.

Stopping in WIN386 may or may not be desirable. For example, the ability of WDEB386 to stop in WIN386 enables VxD developers to single- step through the VxD in question. However, an application or device driver programmer using WDEB386 because of its "protected mode awareness" may have no interest in what WIN386 is doing. In any case, recognizing the system component associated with the current execution stream is a crucial step in using WDEB386 effectively.

Using the Dot Commands

Probably the most interesting (and confusing) part about using WDEB386 concerns the "dot" commands, which are commands preceded by a period. One of the causes of the confusion is that unless the debugging version of WIN386.EXE is installed, most of the dot commands are unavailable. For example, if the following message is displayed while Windows is running in enhanced mode
Win386 not loaded, not debug version, or not responding
it most likely means that the retail version of WIN386.EXE is installed. For more information on installing the debugging version of WIN386, query on the words:
prod(winddk) and wdeb386
Additionally, this message will always appear if WDEB386 is used when Windows is in standard mode.

Dot Dump Commands

Conceptually, the dot commands are "external" commands, or commands that operate on data structures and operations specific to the Windows environment. For example, the "D" (dump) command displays memory locations as would be expected from a debugger, but the ".DG" command displays Windows global heap information in much the same way as the HEAPWALK application.

Most of the .Dx commands do not require the debugging version of WIN386.EXE, and are also available under standard mode. The remainder of the commands described in this article require both the debugging version of WIN386.EXE and enhanced mode operation. Once EVERYTHING is installed correctly, the ".?" help command should provide an online quick reference of the dot commands.

One important distinction that should be made is the difference between the "K" and the ".DS" commands. The "K" command will walk the Windows stack as long as the debugger is stopped in Windows-based application or dynamic-link library (DLL) code. However, if the debugger is tracing through WIN386 code, the "K" command will not produce any useful output. For this reason, the ".DS" command has been provided to display the WIN386 stack. This is another demonstration of the importance of "knowing what is running," as mentioned previously in this article.

Dot VM Commands

WDEB386 was originally designed to debug the enhanced mode layer of Windows; therefore, there may be situations in which the debugger is stopped in the middle of WIN386. For example, if execution is halted using CTRL+ALT+SYS RQ, the machine will not stop immediately at the instruction that was running, but rather at a breakpoint in WIN386 code. Thus, the general registers do not normally contain anything that will be of any use to a developer trying to debug a driver or an application.

However, the operating status of the current virtual machine can be displayed by using the .Vx commands. For example, ".VM" will display the status flags, register contents, current instruction, and a portion of the stack of the current VM. Typing ".VL" will produce a list of all of the VMs in the system. These commands can be used to get an overview of the application, DLL, MS-DOS, or BIOS execution state, as opposed to the state of WIN386.

Dot Memory Commands

The .Mx commands display advanced information on the state of memory. Many of the functions print internal WIN386 information in a more readable format. Two commands that are immediately useful are ".ML" and ".MP". These commands convert addresses from linear to physical, and vice versa.

Dot Trace Commands

The ".T" and ".S" commands provide for keeping interrupt trace information. The trace entries describe what interrupts have occurred, the VM block address, and the interrupted instruction address. These commands can be extremely helpful in tracking down problems (bugs) that do not produce immediate symptoms.

Dot Device Commands

WIN386 and WDEB386 provide the ability for an individual VxD to display information about its own operating state. In general, the user can request this debugging information from a VxD by typing .name at the WDEB386 prompt, where "name" is the name of the VxD. For example, typing .VDMAD produces information about the state of the virtual DMA device.

Issuing a dot device command will cause VMM to send a "Debug_Query" message to the VxD. The VxD is not required to do anything in response to this message, and in fact many VxDs do not produce any debugging output. In general, output produced by VxDs in this manner is not documented, and is only provided as a means of debugging the VxD in question. VxD developers may want to take advantage of this mechanism to display important data structures that define the state of the virtual device.

Dot Command Summary

The dot commands are summarized in Section 9.6 (page 9-48) of the "Microsoft Windows Software Development Kit Tools" manual. An online quick reference screen is available with the ".?" command.

Note: A number of the dot commands are not documented in the SDK tools manual. For example, the format of the dot device command is described, but the actual output produced by specific virtual devices is not given. There are a number of reasons for this:

  • The output produced by the dot commands usually is not produced by the WDEB386 debugger, but rather by components of WIN386. These components are being revised and updated more dynamically than the debugger, and so the information produced by these components is likely to change.
  • The output is often very specific information about the VxD itself, and would not normally be useful in a typical debugging situation.

  • Microsoft Windows Device Development Kit (DDK) for Windows 3.0
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