Setting breakpoint using debug registers

[Vinay]

I went through the specifications of debug registers in intel manuals...It says we can stop the program execution by setting either instruction break point or data breakpoint.. I tried both but breakpoints are not being hit..
I have mentioned the following specification relevant to this..

Say i am setting 4 instruction breakpoints..
DR0 = 0xfffe58a0
DR1 = 0xfffe6860
DR2 = 0xfffe67c0
DR3 = 0xfffe0c68

Debug registers DR4 and DR5 are reserved. So i am not using them.
DR6 is status register, i am not setting it.

After going through the specs, i am setting DR7 = 0x000007FF(I have enables all instruction breakpoints).

But during program execution i am not able to hit the these instruction breakpoints. Its true with data breakpoints also.

Please guide and any help would be appreciated

[Combuster]

Have you tried it on real hardware? The VMs generally lack support for it altogether and Bochs need a compile-time switch to support it.

[Vinay]

Yes, i have tried it on real hardware..

[qw]

This may sound like a dumb question but are you sure that your code is actually executing at the given addresses?

BTW The first two addresses have nine xdigits instead of eight.

[Vinay]

Yes, i am sure that the code is executing at the addresses specified..

Sorry for mentioning wrong DR0 and DR1.. Didn't notice while posting.
It should be
DR0 = 0xfffe58a0
DR1 = 0xfffe6860

[iansjack]

Are those addresses that you are putting in the debug registers physical addresses?

[Vinay]

These are linear addresses...
Since i am not using paging, linear address is same as physical address...

[iansjack]

A really silly question, no doubt, but you don't mention your exception handler. Are you sure this is set up correctly?

[Vinay]

Yes everything is set up properly...
I have manually set breakpoint at DB handler also...its not hitting

[iansjack]

The only other potential problem that I can think of is that the registers aren't pointing to the first byte of the instruction (including any prefix).

[Combuster]

are you sure that your code is actually executing at the given addresses

i am sure that the code is executing at the addresses specified

i am not using paging

0xfffe58a0

That's definitely not your code. It's BIOS space. What are you trying to do there?

[Vinay]

Thanks for the replies...I am still not able to find solution, this is my code only which i have for micro controller operation..

[jbemmel]

Did you try inserting a debug breakpoint instruction (int3) in your code, to test if your debug handler gets called?
It's a different exception vector (#BP 3 instead of #DB 1), but it's easier to setup and very similar

[JamesM]

Thanks for the replies...I am still not able to find solution, this is my code only which i have for micro controller operation..

If you're not using paging, then like Combuster said that address is in the BIOS region. Are you certain that it points to the *first byte* of a valid instruction? And that the CPU isn't operating in real mode by that point?

[Vinay]

I am sure that CPU is operating in protected mode, and now my query is how to check it points to the first byte of the instruction?