The Conversion from the linear address to physical address is done by a separate processor or a MMU
In architecture Independent model, page conversion is a three level process which takes four steps
1st part is used as an index in the page directory which refers to the page middle directory
2nd part serves as an index to the page middle directory, there it refers to the page table
3rd Part used as an index to the page table, there it refers to the physical memory
4th Part of the address gives the offset within the selected page of memory.
The x86 model supports a two level conversion of linear address to physical address.
Highmem support helps upto physical memory size of 4GB being the address space of 32bit address, but Intel added four additional address pins to the Pentium Pro and created the physical address extension (PAE). Since the two level conversion supported only 32 bit addresses, the PAE has the control of supporting three level address conversion as in the Linux Architecture Independent Model.