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max wrote:Hey CrafterMan24,

please try to explain what exactly you don't understand, so we can try to help you better. Have you understood how the virtual to physical translation is done (how the calculation works)?

Greets,
Max

CrafterMan24 wrote:I just understand page tables on page directory, enabling paging and loading page directory... All others... Umm. I understand nothing except them

virtual address = (directory_index * 1024 * 0x1000) + (table_index * 0x1000);
virtual address = (41 * 1024 * 0x1000) + (23 * 0x1000);
virtual address = 0x0A417000;

max wrote:

CrafterMan24 wrote:I just understand page tables on page directory, enabling paging and loading page directory... All others... Umm. I understand nothing except them

Hm. So what do you think that these are used for?

When you load a page directory and paging is enabled, then this is what the MMU uses to translate any virtual address to a physical address. A virtual address is what executing code "sees" when accessing memory. A virtual address is translated to a physical address via the respective index in the paging directory and page table where the page is mapped into. If you place the address of a physical page (say, 0x1A841000) into page directory index 41, page table index 23, then the address will be:

Code: Select all

virtual address = (directory_index * 1024 * 0x1000) + (table_index * 0x1000);
virtual address = (41 * 1024 * 0x1000) + (23 * 0x1000);
virtual address = 0x0A417000;

When the executing code now accesses the memory at virtual 0x0A417000, it will actually write to the physical page 0x1A841000.

SpyderTL wrote:A modern OS is responsible for managing all of the shared resources on the machine, and providing safe access to those resources to any applications running on that machine. Memory is just one of those resources.

One of the easiest ways to share resources between applications is to prevent "direct" access to those resources, and instead force the application to access "virtual" resources. That way, the application can assume that it is the only application running on the machine, and can access any resource it wants.

For memory, this can be done by programming the MMU to look up actual memory addresses in a table. The OS is responsible for keeping this table up to date.

So, now every application thinks that it can access 4GiB of memory (in 32-bit mode). But you may only have 1 GiB of actual memory. So, the OS can move some memory that is not being used to the hard drive by catching a "Page Fault" exception, which is thrown by the MMU if an address requested by an application is not listed in the table (or if it is listed, but has been flagged as "not available").

So, when this exception occurs, the OS is responsible for finding an area in actual memory that is not currently being used, and adding it to the table, giving it a new address that matches the address requested by the application. If the requested address is already in the table, then the OS may also need to move some memory from the hard drive back into memory.

Once all of this is done, the OS returns control back to the application, which attempts to access the memory again. All of this happens without the application even noticing. All the application sees is that it read some data from a memory address.

Hopefully, this clears things up a bit. It helps to think of each application running in its own virtual machine, and the OS just manages all of the virtual machines on one real machine, at least for me.

SpyderTL wrote:All of this happens without the application even noticing.

embryo2 wrote:I don't want to convince anybody not to use paging, but I want to remind that the paging is just one of possible memory management strategies and it's disadvantages should be considered very carefully. And as a result - alternate memory management strategies also should be explored.

Even if an application haven't noticed anything the user really notices that the application is almost dead. And (at least under Windows) if there's not enough memory then the whole OS goes "down". Very unpleasant situation.

BASICFreak wrote:

embryo2 wrote:I don't want to convince anybody not to use paging, but I want to remind that the paging is just one of possible memory management strategies and it's disadvantages should be considered very carefully. And as a result - alternate memory management strategies also should be explored.

Well, the disadvantage of paging does NOT equal the disadvantage of not paging.

Without paging, you will never get into Long Mode, access >3GB of RAM, run multiple processes at once (yes segmentation exists on SOME processors so last is not 100% true) [just lost 70% of my systems resources, where's my 8GB of ram?]

With paging, you have to keep track of tables (1-4 layers depending on page size and type). [not much of a disadvantage IMO]

These are only the first things I thought of...

Even if an application haven't noticed anything the user really notices that the application is almost dead. And (at least under Windows) if there's not enough memory then the whole OS goes "down". Very unpleasant situation.

In my experience windows will not go down, paging to HDD will start and slow the system to a crawl. When the system is slow the app trying to use the pages on the HDD will be waiting for data and not responding the "I'm Alive" ping - and windows sees the program as crashed, if you touch it it closes if you don't it may stay open until its data arrives.

Or if its out of video memory you may have a full crash (I blame NVidia)



But, yes paging to HDD should be avoided! If you must, check speeds on disks and find the quickest one, and only cache low priority - hardly used - data

BASICFreak wrote:Well, the disadvantage of paging does NOT equal the disadvantage of not paging.

BASICFreak wrote:Without paging, you will never get into Long Mode

BASICFreak wrote:access >3GB of RAM

BASICFreak wrote:run multiple processes at once (yes segmentation exists on SOME processors so last is not 100% true) [just lost 70% of my systems resources, where's my 8GB of ram?]

BASICFreak wrote:With paging, you have to keep track of tables (1-4 layers depending on page size and type). [not much of a disadvantage IMO]

BASICFreak wrote:In my experience windows will not go down, paging to HDD will start and slow the system to a crawl.

BASICFreak wrote:sometimes you must wait patiently