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Hardware breakpoints

Note: The following description, and the code, were written in the year 2000. This code only runs on Windows, and only 32-bit. Also, I haven't tried it on any version of Windows past Windows XP, or any recent version of Visual C++.

This is a debugging helper class which lets you set breakpoints on the fly from within code. This is mainly useful for the case where you have a variable that you know is getting trashed, but you have no idea who is trashing it. You can cause the debugger to break in at the very moment the variable is changed. The really cool thing is that this makes use of the Intel Pentium's built-in debug registers, which means that it really will stop no matter what code is executing, even if it's down in the NT kernel, in a different thread, or whatever. This code also apparently works fine on some AMD CPUs.

(Visual C++ has the ability to set a breakpoint when a variable's value changes, but these breakpoints can be very hard to use, especially on local variables.)

Usage

The class is called HardwareBreakpoint. If, for example, you have a DWORD called x and you want to break the next time it's written to, do this:

    DWORD x = 1;

    HardwareBreakpoint bp;
    bp.Set(&x, sizeof(x), HardwareBreakpoint::Write);

As your code continues to execute, if Visual C++ stops at a location where you didn't have a VC++ breakpoint set, it's possible that the HardwareBreakpoint triggered. If it did, the assembly instruction immediately before the instruction pointer is the one that caused it to trigger.

The breakpoint will be cleared automatically when the HardwareBreakpoint object falls out of scope. Or, to clear it explicitly, do this:

    bp.Clear();

You can also break the moment any code even tries to read the value of the variable x! (Actually, this will break when someone tries to read or write it.)

    bp.Set(&x, sizeof(x), HardwareBreakpoint::Read);

Notes

I've tested this code on versions of Windows up to and including Windows XP. I have only tried it on Intel chips, but another developer told me that it worked for him on the AMD Athlon II X2 250.

There are certain limitations when the breakpoint is triggered inside system code (these same limitations apply to hardware breakpoints set by Visual C++ or any other debugger). Some system code runs in ring 3 (with user privileges), and other system code runs in ring 0. If a hardware breakpoint triggers inside ring 0 code, the debugger will stop, but not exactly when the data write happens -- it will stop after execution transitions back to ring 3 code. In practice, this is not a problem at all: When the breakpoint triggers, it's usually quite easy to figure out what happened.

You can do bp.Clear() from the QuickWatch dialog to clear a breakpoint from within the debugger.

The second parameter is the number of bytes to watch. This must be either 1, 2, or 4. Also, the variable being watched must be aligned appropriately (e.g. if you're watching 4 bytes, they must be DWORD-aligned). If you don't do this, you'll get an error when you try to set the breakpoint. There is a limit of 4 hardware breakpoints. Because of this (and also because of common sense), don't check in code that uses breakpoints -- just write the code temporarily to find bugs, but delete it before checking in your source into your source control system.

It's important to pass in the address that you actually want to watch. Consider these two similar but different cases:

    long *my_array;
    my_array = new long[3];
    
    HardwareBreakpoint bpArrayPtr, bpArrayFirstElement;
    
    // break if someone changes the POINTER
    bpArrayPtr.Set(&my_array, sizeof(long*), HardwareBreakpoint::Write);
    
    // break if someone changes the FIRST ELEMENT POINTED TO
    bpArrayFirstElement.Set(&my_array[0], sizeof(long), HardwareBreakpoint::Write);

How does it work?

The Intel x86 CPUs have some special registers which are intended for debugging use only. By storing special values into these registers, a program can ask the CPU to execute an INT 1 (interrupt 1) instruction immediately whenever a specified memory location is read from or written to. (They can also stop when a memory address is about to be executed as code, but my HardwareBreakpoint class doesn't use that functionality.)

INT 1 also happens to be the interrupt that's executed by the CPU after a debugger asks the CPU to single-step one assembly line of the program. And by good fortune, when Visual C++ encounters an INT 1 that it wasn't expecting to see (as is the case when a HardwareBreakpoint breakpoint is triggered), it responds gracefully, stopping the debugger at the appropriate instruction.