We all know that data retrieval will be fast if the data pages are found in RAM .If the data pages are not in RAM, they are fetched into RAM from the disk. This causes a physical IO .The page remains in the RAM until it’s again kicked off to Disk.
But the process and threads do not access the Physical memory (RAM) directly .Instead the RAM is accessed indirectly through Virtual Memory or Virtual address space (VAS) pointers. On a x86 operating system the number of such pointers in virtual memory that can point to physical memory is 4,294,967,296 (2^32) .This is equal to 4 GB .Out of this 4GB VAS pointers, 2GB worth of pointers are located in the Kernel address space and remaining 2GB in the User Address Space .It’s this 2GB of user address space which is used by the Processes and threads for their use and to map it to RAM. Other 2GB Kernel Address space is also mapped to RAM for the OS routines and APIs .So, normally on a 32 bit windows OS, SQL Server will use 2GB RAM (1.66GB Buffer Pool region and 384MB Mem2Leave region) .
But what’s the need for Virtual memory when we have Physical memory and it is the real memory .Let me correlate this to a smart Bank. The bank started with $5000.A customer deposited $1000 and the bank will return $1100 after a year .after 1 month, Another customer deposited $2000 for 1 year and the Bank will return $2200 after a year. So the bank has now $8000 for around 2 years .Then someone took a loan of $3000 for 1 year and the bank will get $4000 after a year .In between if the earlier 2 clients want to withdraw their deposit before time, they can pay the penalty and Bank has sufficient money to give back from the initial investment .In reality the banks or the moneylenders keep revolving the money which they might not even have.

I hope you have some idea now even though the example is not so good . The OS also works like this .It assures every process 4GB of memory on a 32 bit WIndows OS.Right now there are 118 processes running on my laptop .If we go by this fact then the OS is ensuring 472 GB RAM to the processes .But I have only 2GB RAM on this laptop. That’s where the Virtual memory comes into picture .This 476 GB is actually a virtual memory address space and nothing else ; which does physically not exist .OS memory managers maps this virtual memory to Physical memory (RAM) .During this process the Page file on the disk is also used if a thread needs more physical memory than available .


Let’s skip discussing about AWE, /3GB, /USERVA and PAE for now as it will divert us from the topic which is to know how the Virtual addresses get translated to Physical addresses in RAM.Address translation is the process of translating the virtual memory to physical memory.Every Virtual Address has 3 components:
The Page Directory Index : For each process the OS memory manager creates this directory to use it to map all the page tables for the process.The address of this directory in stored somewhere in the address space called as KProcess Block (Kernal Process Block). To keep this subject less complex I will not explain what Kprocess Block is .The CPU keeps track of this Page Directory Index via a register called as CR3 or Control Register 3 .This Register also resides in KProcess Block .So the CPU’s MMU knows where the Page Directory Index is located with the help of This register (MMU: the first 10 bits of the address space pointer has Page Directory Index value (there are a lot of page directory entries).This tells Windows which page table to use to locate the physical memoryassociated with the address.
The Page Table Index : The second 10 bits of a 32-bit virtual address provide an index into this table and indicate which page table entry (PTE) contains the address of the page in physical memory to which the virtual address is mapped.

The Byte Index: the last 12 bits of a 32-bit virtual address contain the byte offset on the physical memory page to which the virtual address refers. The system page size determines the number of bits required to store the offset. Since the system page size on x86 processors is 4K, 12 bits are required to store a page offset (4,096 = 2^12).

Let’s summarize it now:

  1. The CPU’s Memory Management Unit locates the page directory for the process using the special register mentioned above.
  2. The page directory index (from the first 10 bits of the virtual address) is used to locate the (P)age(D)irectory(E)ntry that identifies the page table needed to map the virtual address to a physical one.
  3. The page table index (from the second 10 bits of the virtual address) is used to locate the PTE that maps the physical location of the virtual memory page referenced by the address.The PTE is used to locate the physical page. If the virtual page is mapped to a page that is already in physical memory, the PTE will contain the page frame number (PFN) of the page in physical memory that contains the data in question. If the page is not in physical memory, the MMU raises a page fault, and the Windows page fault–handling code attempts to locate the page in the system paging file. If the page can be located, it is loaded into physical memory, and the PTE is updated to reflect its location. If it cannot be located and the translation is a user mode translation, an access violation occurs because the virtual address references an invalid physical address. If the page cannot be located and the translation is occurring in kernel mode, a bug check (also called a blue screen) occurs.


How the Address translation happens with PAE in place:
Everything is same as above except that:
1)There is a new table which is above PDEs and PTEs .Its Page Directory Pointer Table.
2)The PTEs and PDEs are 64 bit wide as compared to 32 bit wide when PAE is not enabled.

Happy learning!



Abhay Chaudhary

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