OSDev.org

Posted: **Fri Mar 07, 2008 11:39 am**

I, too, suspected that, but it didn't fix the issue. There appears to be something wrong with my descriptor tables according to Bochs' CPU debugger. It's fine, I'm doing a rewrite anyways.

Posted: **Fri Mar 07, 2008 8:08 pm**

Ugh, the rewrite didn't do anything! Here's the output from Bochs:

Code: Select all

00025849647d[CPU0 ] PDE: entry not present
00025849647d[CPU0 ] page fault for address 80000011 @ 00101729
00025849647d[CPU0 ] exception(0x0e): error_code=0002
00025849647d[CPU0 ] interrupt(): vector = 14, INT = 0, EXT = 1
00025849647d[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x0e)
00025849647d[CPU0 ] exception(0x0d): error_code=0072
00025849647d[CPU0 ] interrupt(): vector = 8, INT = 0, EXT = 1
00025849647d[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x08)
00025849647d[CPU0 ] exception(0x0d): error_code=0042

So I have to inspect the paging code and IDT code...again. Terrible.

Posted: **Sat Mar 08, 2008 7:28 pm**

I had the same problem when following JamesM's tutorials - for some weird reason I kept page faulting at 0x80000011, which is the CR0 value.

I ended up scrapping that code and writing my own, but it wouldn't hurt to step through the code in the Bochs debugger to try to figure out how it's happening.

Posted: **Sat Mar 08, 2008 8:28 pm**

pcmattman: I've scrapped the code several times, tried two different versions of it, written my own, and used the one off of OSDever.net. None of them worked. With this latest rewrite of the code, there are some new messages in the Bochs debug log, and none about the gates. After going through the code with the debugger I've come to the same place I was before: when the paging flag is set in CR0, the triple fault occurs

"mov eax, cr0" is the disassembly of the code at EIP

Bochs debug output, source code.

Posted: **Sun Mar 09, 2008 7:24 am**

your switch_page_directory function is fine, but the part in it that enable's paging (apparently) causes the problem. move it out of that function, and into a new function, say, enable_paging.

(it's in ASM)

Code: Select all


[global _enable_paging]
_enable_paging:
       mov cr0, eax
       or eax, 0x80000000
       mov eax, cr0
       ret

should work then.

Posted: **Sun Mar 09, 2008 10:17 am**

That didn't work.

According to Bochs, my paging table entries don't have the PRESENT bit set, but I can't see anything wrong with them...

Posted: **Sun Mar 09, 2008 3:32 pm**

If you read the Intel Manuals (Volume 3A, exception reference section) you find that CR2 holds the virtual address of the faulting address. Therefore when your CR2 value when Bochs triple faults is 0x80000011, you know something weird is going on.

You don't have an entry for 0x80000000 in your page directory or tables, so Bochs complains about the present bit

.

Posted: **Sun Mar 09, 2008 4:52 pm**

Yes, I knew that CR2 contained the faulting address. That's why I said that CR2==CR0 was weird. What I'm struggling with is getting the damned thing to work. Might CPaging::SwitchPageDir really be the problem?

Posted: **Sun Mar 09, 2008 11:18 pm**

TBH, I actually never got JamesM's code working - it did exactly the same thing.

One thing that *might* work is doing the write to CR0 in pure assembly, so the compiler doesn't try to inject extra information into the assembly.

Posted: **Mon Mar 10, 2008 5:49 am**

What's this:
00028889748i[CPU0 ] >> mov eax, dword ptr ss:[ebp+0x8] : 8B4508
?
That doesn't look like "mov eax, cr0" to me.

I use the FASM manual as a guideline for the Intel Instruction Set and it says that the mov instruction can be used to transfer a global register to a control register, but it doesn't say anything about using it to copy memory to a control register. The best way is to use the eax register. That always worked for me.

Posted: **Mon Mar 10, 2008 1:10 pm**

When I reboot to Linux, I'll try doing it in full assembly (again). Thanks for the suggestions.

Posted: **Mon Mar 10, 2008 1:12 pm**

Just had a look at the code above (where you enable paging),

Code: Select all

 __asm__ __volatile__ ("mov %0, %%cr3" : : "r" (&dir->tables_phys));

What's the use of the '&' in "&dir->tables_phys"? AFAICS, 'tables_phys' is defined as an array of ints, so using 'dir->tables_phys' should give you the starting address of the array (=the base address of the page directory), which is what you want to put into cr3, don't you? If this piece of code gives you a borked value, it's likely that the mmu complains about invalid page tables. Please let me know if it makes a difference,

cheers
joe

Posted: **Mon Mar 10, 2008 8:38 pm**

pcmattman wrote:TBH, I actually never got JamesM's code working - it did exactly the same thing.

OMG! We are three now

I tried to use JamesM paging code and it didn't work, same problem as both of you. Finally I used a adapted version from the code in osdever, it works.

Cheers,
Gonzalo

Posted: **Tue Mar 11, 2008 2:58 am**

JoeKayzA wrote:Just had a look at the code above (where you enable paging),
Code: Select all
 __asm__ __volatile__ ("mov %0, %%cr3" : : "r" (&dir->tables_phys)); 
What's the use of the '&' in "&dir->tables_phys"? AFAICS, 'tables_phys' is defined as an array of ints, so using 'dir->tables_phys' should give you the starting address of the array (=the base address of the page directory), which is what you want to put into cr3, don't you? If this piece of code gives you a borked value, it's likely that the mmu complains about invalid page tables. Please let me know if it makes a difference,

cheers
joe

You know I think he's right! tables_phys IS the address you're supposed to be putting in there, not &tables_phys, which is the address of the address...
OMG. If this is the way it's in JamesM's tutorial maybe he should change it. Weird... I think I saw this problem before somewhere...

I'm having deja-vu.
Anyway, this kind of bugs usually make me laugh for days...

You can't go wrong with C/C++! Yes you can! Always!

Posted: **Tue Mar 11, 2008 11:03 am**

That actually sounds like a solution...wow...thanks! I'll be sure to try it first thing once I'm back in Linux.

OSDev.org

Paging Triple Fault

original code looks fine, except the enable paging part.