What does this little bit of x86 doing with cr3?

Question

So, I'm disassembling a bit of code for fun and no profit (other than the joy of knowledge, that is), and I come across this bit:

mov eax, cr3
mov cr3, eax

Is this doing anything other than just masturbating with cr3? If so, what? This is x86 low-level (bios/firmware/before boot loader) intialization code. We haven't even setup up caches yet.

Answer 1

It is flushing the TLBs (Translation Lookaside Buffers) by loading cr3 with itself.

Intel even mentions the code in their "Intel 64 and IA-32 Architectures Software Develoment Manual Volume 3A - System Programming Guide".

mov EAX,CR3  ; invalidate the TLB
mov CR3,EAX  ; by copying CR3 to itself

You can find that and many more handy manuals at:

http://www.intel.com/products/processor/manuals/index.htm