NASM - The Netwide Assembler

NASM Forum => Programming with NASM => Topic started by: junior_study on July 21, 2012, 01:59:13 PM

Title: can I get cr3's value like this?
Post by: junior_study on July 21, 2012, 01:59:13 PM

hi:

       this instruction   ?   movl  %cr3  ,  %eax   
       can  this indtruction  load  cr3's value into  eax?

thank you!

Title: Re: can I get cr3's value like this?
Post by: Frank Kotler on July 21, 2012, 05:07:02 PM

No, not in Nasm syntax. Try:

Code: [Select]

mov eax, cr3
... and not in user mode in a protected OS. A "mov" into or out of a control register is a "privileged instruction" - won't work in ring 3. From "real real-mode dos" (or "write my own OS mode", yes. Probably not meaningful until you've loaded something into it... and not effective until you've enabled paging (bit 31 in cr0, I think). Possibly in a kernel module...

You probably want to study the Intel/AMD manual(s), and/or http://www.sandpile.org and/or http://www.osdev.org More of a "senior study", I think. :)

Best,
Frank