Page 34 - DCAP408_WEB_PROGRAMMING

Windows Programming

Notes 0x13453###. As pointed previously, lower 12 bits are used to accumulate page protection
information and other information required by an OS. These are out of the scope of our current
discussion so I have shown them as ###. The upper 20 bits displays the Page Frame Number or
PFN which is the physical address of a 4KB aligned page. The actual physical address
corresponding to the PFN will be 0x13453000. Let us now perform the translation:
0x2034AC54

 0x2034AC54 can be displayed as 0010000000 1101001010 110001010100
 The upper 10 bits which are 0010000000 provides the index into page directory.
Transforming to hexadecimal, the upper 10 bits give a value of 0x080

 From CR3, we know the Page Directory is situated at physical address 0x13453000 and
from discussion above we also know that Page Directory is situated at logical address

 0xC0300000
 So 0xC0300000 [0x080] will give the address of page table which is P_PT. From the table
above, we can observe that this address is displayed by page table at logical address
0xC00001000 (or physical address 0x45045000). Now we utilize next 10 bits i.e. 1101001010
(or 0x34A) to index into the page table.
 The address 0xC00001000 [0x34A] will provide us the physical address of a 4KB page
which is 0x34005000 from the table above.
 The number displayed by lower 12 bits, which is 110001010100 (or 0xC54), is used to point
to the actual byte on the 4KB page located at 0x34005000. The final physical address
corresponding to 0x2034AC54 comes out to be 0x34005C54
I will leave the address translation of other 3 addresses as an exercise to the reader. Once you do
that translation, you will appreciate why the PDB entry is stored at index 0x300 in the table
above and it causes processor to consider page directory as a page table during address translation.
Also this translation will provide you more information on why this specific layout was selected
by windows designers.

Task Make distinction between logical address and physical address.

Self Assessment

Fill in the blanks:
14. Windows provides lower 2GB (or 3GB relying upon boot.ini switch) logical address space
of a process to user mode and upper 2GB (or 1GB relying upon boot.ini switch) to Windows
........................ .

15. The prerequisite for storing the page tables in ........................ is that a rouge consumer
mode application should not be able to influence the page tables.

2.5 Summary

 Memory management in Windows operating systems has developed into a affluent and
classy architecture, competent of scaling from the minute embedded platforms all the way

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