Paging: Difference between revisions

INVLPG is an instruction available since the 486 that invalidates a single page table entry in the TLB. Intel warns that this instruction may be implemented differently on future processors. INVLPG modifies no flags.

 

invlpg [0]

 

__asm__ volatile("invlpg %0"::"m" (*(char *)addr));

 

===Example===

Say I loaded my kernel to 0x100000. However, I want it mapped to 0xc0000000. After loading my kernel, I initiate paging, and set up the appropriate tables. (See [[Higher Half Kernel]]) After [[Identity Paging]] the first megabyte, I start to create my second table (ie. at entry #768 in my directory.) to map 0x100000 to 0xc0000000. My code could be like: <br /><br />

 

mov eax, 0x0

mov ebx, 0x100000

jmp .fill_table

.end:

 

==Enabling==

Enabling paging is actually very simple. All that is needed is to load CR3 with the address of the page directory and to set the paging bit of CR0.

 

mov eax, [page_directory]

mov cr3, eax

or eax, 0x80000000

mov cr0, eax

 

To enable PSE (4 MiB pages) the following code is required.

 

mov eax, cr4

or eax, 0x00000010

mov cr4, eax

 

==Physical Address Extension==

 

Many prefer to map the last PDE to itself. The page directory will look like a page table to the system. To get the physical address of any virtual address in the range 0x00000000-0xFFFFF000 is then just a matter of:

void * get_physaddr(void * virtualaddr)

{

return (void *)((pt[ptindex] & ~0xFFF) + ((unsigned long)virtualaddr & 0xFFF));

}

 

To map a virtual address to a physical address can be done as follows:

void map_page(void * physaddr, void * virtualaddr, unsigned int flags)

{

// or you might not notice the change.

}

Unmapping an entry is essentially the same as above, but instead of assigning the <code>pt[ptindex]</code> a value, you set it to 0x00000000 (i.e. not present). When the entire page table is empty, you may want to remove it and mark the page directory entry 'not present'. Of course you don't need the 'flags' or 'physaddr' for unmapping.