PRB: Debuggers Cannot Reliably Use Debug Register Breakpoints

• Microsoft Win32 Software Development Kit (SDK) version 4.0

SYMPTOMS

Debug register breakpoints are breakpoints that trigger on write or read of a specific memory location. If you use debug register breakpoints in application debuggers such as CodeView, Visual C++, or Windbg, other applications in the system could fault.

CAUSE

Debug registers are not saved and restored on a per application basis in Windows 95; they are global to the system.

A debug register running at the same address (but in another memory context) could trigger that debug breakpoint. Because there may not be a debugger registered to handle the breakpoint in the second application, Windows 95 interprets this as an unhandled exception. In most cases, this causes Windows 95 to terminate the second application, even though it had nothing to do with the problem.

RESOLUTION

To work around this problem, use a system level debugger such as Soft-ICE or WDEB386, or avoid using debug register breakpoints.

STATUS

This behavior is by design.