Register operations in AMD-64

gobi_dgm · 06-16-2006, 04:22 AM

Hi ,
I have to enable the alignment-check exception in amd x86_64 machine. I read the AMD manual volume 2 and got something useful..

An #AC exception occurs when an unaligned-memory data
reference is performed while alignment checking is enabled.
After a processor reset, #AC exceptions are disabled. Software
enables the #AC exception by setting the following register bits:
􀂄 CR0.AM=1.
􀂄 RFLAGS.AC=1.
When the above register bits are set, an #AC can occur only
when CPL=3. #AC never occurs when CPL < 3.

It asks me to reset the flags mentioned above. I dont know how to do this and i doubt whether any application is allowed to do so..
i need your help in doing the same..

thanks in advance
gobi

paulsm4 · 06-16-2006, 10:29 PM

*Why* do you "have to enable alignment checking"???

Knowing a little of your problem's *context* might help give a better solution.

Your .. PSM

gobi_dgm · 06-18-2006, 11:06 PM

Hi,
we are trying to understand what are the possible changes to be done to a codebase while porting it from 32-bit to 64-bit machine. So i thought it will be better to have a practical knowledge about 64-bit machines before doing that. Since misalignment is the major problem while migrating, i thought of enabling it in our 64-bit server..
Please help me in doing the same..

Thanks in advance
Gobi

paulsm4 · 06-20-2006, 12:01 AM

Hi -

As far as *how* to do it, most C/C++ compilers (including gcc) permit in-line assembly code. The easiest way is to just try it and see if the OS lets you get away with modifying that particular flag. The worst that could happen is you might crash your system and have to reboot ;-)

As far as misalignment being a "major problem", I think it's probably actually among the *least* of your worries. I quote from the AMD Porting FAQ (listed below:

Quote:

What are the memory alignment requirements for code and data in each of these
operating systems?

There are no alignment requirements. x86 architecture has never had any alignment
requirements and the 64-bit extensions have not added any. However, the ABI for
64-bit software has been designed to improve performance by striving for natural
alignment of most memory operands...

The article goes on to say that the biggest impact (if any) would be on performance.

Finally, here are some good references (most from the AMD site itself) that might give you some additional information:
- http://www.devx.com/amd/Article/16520
- http://www.devx.com/amd/Door/21585
- http://www-128.ibm.com/developerwork.../l-port64.html
- http://www.amd.com/us-en/assets/cont...orting_FAQ.pdf

'Hope that helps .. PSM