How is PAE implemented in Linux?

Question

There are only 36 physical address lines but a page table entry is 64 bit long. How does the CPU check if the address is valid ? How does Linux OS map the same virtual address in different processes to different physical addresses?

Here is the wikipedia article about physical address extension(PAE). http://en.wikipedia.org/wiki/Physical_Address_Extension .

Answer 1

There are only 36 physical address lines but a page table entry is 64 bit long. How does the CPU check if the address is valid?

This is actually done by a piece of hardware called the memory management unit (MMU), which handles virtual-to-physical address translation. In terms of an address being "valid", each process has it's own unique address space (thus implementing a virtual memory scheme), so any address is technically valid. Remember, a process can allocate more memory than is physically available.

The MMU uses a translation lookaside buffer (TLB) to quickly decode the virtual address into a physical one, held within a page table.

How does Linux OS map the same virtual address in different processes to different physical addresses?

This is again due to each process having a unique virtual address space, which is fundamental for implementing virtual memory. A virtual address for a given process is mapped to some physical storage hardware (RAM, disk, etc...), but the mapping is done at run-time by the operating system & MMU.