Page 264 - DCAP403_Operating System

Unit 13: Case Study: Linux

The page table is accessed using the virtual page frame number as an offset. Virtual page frame Notes
5 would be the 6th element of the table (0 is the fi rst element).

To translate a virtual address into a physical one, the processor must first work out the virtual
addresses page frame number and the offset within that virtual page. By making the page size
a power of 2 this can be easily done by masking and shifting. Looking again at Figures 13.2 and
assuming a page size of 0x2000 bytes (which is decimal 8192) and an address of 0x2194 in process
Y’s virtual address space then the processor would translate that address into offset 0x194 into
virtual page frame number 1.
The processor uses the virtual page frame number as an index into the processes page table to
retrieve its page table entry. If the page table entry at that offset is valid, the processor takes the
physical page frame number from this entry. If the entry is invalid, the process has accessed a
non-existent area of its virtual memory. In this case, the processor cannot resolve the address and

must pass control to the operating system so that it can fix things up.

Just how the processor notifies the operating system that the correct process has attempted
to access a virtual address for which there is no valid translation is specific to the processor.

However the processor delivers it, this is known as a page fault and the operating system is
notified of the faulting virtual address and the reason for the page fault.

Assuming that this is a valid page table entry, the processor takes that physical page frame
number and multiplies it by the page size to get the address of the base of the page in physical
memory. Finally, the processor adds in the offset to the instruction or data that it needs.
Using the above example again, process Y’s virtual page frame number 1 is mapped to physical
page frame number 4 which starts at 0x8000 (4 x 0x2000). Adding in the 0x194 byte offset gives

us a final physical address of 0x8194.
By mapping virtual to physical addresses this way, the virtual memory can be mapped into the
system’s physical pages in any order. For example, in Figure 13.2 process X’s virtual page frame
number 0 is mapped to physical page frame number 1 whereas virtual page frame number 7 is
mapped to physical page frame number 0 even though it is higher in virtual memory than virtual
page frame number 0. This demonstrates an interesting byproduct of virtual memory; the pages
of virtual memory do not have to be present in physical memory in any particular order.

13.5.1 Demand Paging

As there is much less physical memory than virtual memory the operating system must be careful

that it does not use the physical memory inefficiently. One way to save physical memory is to
only load virtual pages that are currently being used by the executing program. For example, a
database program may be run to query a database. In this case not the entire database needs to
be loaded into memory, just those data records that are being examined. If the database query is
a search query then it does not make sense to load the code from the database program that deals
with adding new records. This technique of only loading virtual pages into memory as they are
accessed is known as demand paging.
When a process attempts to access a virtual address that is not currently in memory the processor
cannot find a page table entry for the virtual page referenced. For example, in Figure 13.2 there is

no entry in process X’s page table for virtual page frame number 2 and so if process X attempts
to read from an address within virtual page frame number 2 the processor cannot translate the
address into a physical one. At this point the processor notifies the operating system that a page

fault has occurred.
If the faulting virtual address is invalid this means that the process has attempted to access a
virtual address that it should not have. Maybe the application has gone wrong in some way, for
example writing to random addresses in memory. In this case the operating system will terminate
it, protecting the other processes in the system from this rogue process.

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