Page 30 - DCAP408_WEB_PROGRAMMING

Windows Programming

Notes As discussed above, the whole address space is divided in 4KB pages. So when a PDE or PTE is
used, its upper 20 bits gives a 4KB page aligned address and lower 12 bits are utilized to store the
page defense information and some other housekeeping information needed by an OS for
proper functioning.

Notes The upper 20 bits which shows the actual physical address are called Page Frame
Number (or PFN). Details on defense bits and other bits in the lower 12 bits can be located
in here.

Task Make distinction between page directory and page table.

2.2.1 Windows Page Table Management

In windows every process has its own Page Directory and Page Tables. Therefore windows
assign 4 MB of this space per process. When a process is formed, every entry in Page Directory
includes physical address of a Page Table. The entries in the page table are either valid or
invalid. Valid entries enclose physical address of 4KB page assigned to the process. An invalid
entry includes some particular bits to mark it invalid and these entries are called Invalid PTEs.
As the memory assigned by the process, these entries in Page Table are filled with the physical
address of the assigned pages. You should keep in mind one thing here that a process doesn’t
recognize anything regarding physical memory and it only utilizes logical addresses. The details
of which logical address corresponds to which physical address is handled evidently by Windows
Memory manager and the processor. The address at which the page directory of a process is
positioned in physical memory is pointed to as Page Directory Base address. This Page Directory
Base address is amassed in a special CPU register called CR3 (on x86). On context switch, Windows
loads the new value of CR3 to point to the new process’s Page Directory Base. This manner every
process gets its own division of the whole 4GB physical address space. Certainly the total
memory assigned at one time to all the process’s in a system cannot go beyond the total amount
of RAM + page file size but the method discussed above permits windows to give every process
its own 4GB logical (or Virtual) address space. We call it Virtual Address space since although
the process has the whole 4GB address range obtainable to it, it can only use the memory which
is assigned to it. Compilers can generate a program that depends on the code being at an accurate
location in memory, every time it is executed.

Example: With virtual memory, the process considers, it is at 0x080482a0, but in fact it
could be at physical memory position 0x1000000.

If a process attempts to access an unallocated address, it will obtain an access violation since the
PTE subsequent to that address refers to an invalid value. Also the process can’t assign more
memory than what is obtainable in the system. This way of separating logical memory from
physical memory has many benefits. A process gets a linear 4GB address space so application
programmers don’t have to concern regarding segments and all unlike in old DOS days. It also
permits windows to run multiple processes and let them employ physical memory on a machine
without worrying about them stomping on each other’s address space. A logical address in one
process will never refer to the physical memory assigned to another process (unless they are
utilizing some sort of shared memory). Therefore, one process can never read from or write to
another process’s memory.

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