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
- 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
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
> 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
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
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
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
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
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.