What is Virtual Memory?

A program instruction on an Intel 386 or later CPU can address up to 4GB of memory, using its full 32 bits. This is normally far more than the RAM of the machine. (The 32nd exponent of 2 is exactly 4,294,967,296, or 4 GB. 32 binary digits allow the representation of 4,294,967,296 numbers — counting 0.) So the hardware provides for programs to operate in terms of as much as they wish of this full 4GB space as Virtual Memory, those parts of the program and data which are currently active being loaded into Physical Random Access Memory (RAM). The processor itself then translates (‘maps’) the virtual addresses from an instruction into the correct physical equivalents, doing this on the fly as the instruction is executed. The processor manages the mapping in terms of pages of 4 Kilobytes each - a size that has implications for managing virtual memory by the system.

What are Page Faults?

Only those parts of the program and data that are currently in active use need to be held in physical RAM. Other parts are then held in a swap file (as it’s called in Windows 95/98/ME: Win386.swp) or page file (in Windows NT versions including Windows 2000 and XP: pagefile.sys). When a program tries to access some address that is not currently in physical RAM, it generates an interrupt, called a Page Fault. This asks the system to retrieve the 4 KB page containing the address from the page file (or in the case of code possibly from the original program file). This — a valid page fault — normally happens quite invisibly. Sometimes, through program or hardware error, the page is not there either. The system then has an ‘Invalid Page Fault’ error. This will be a fatal error if detected in a program: if it is seen within the system itself (perhaps because a program sent it a bad request to do something), it may manifest itself as a ‘blue screen’ failure with a STOP code: consult the page on STOP Messages on this site.

If there is pressure on space in RAM, then parts of code and data that are not currently needed can be ‘paged out’ in order to make room — the page file can thus be seen as an overflow area to make the RAM behave as if it were larger than it is.

What is loaded in RAM?

Items in RAM can be divided into:

• The Non-Paged area. Parts of the System which are so important that they may never be paged out - the area of RAM used for these is called in XP the ‘Non-Paged area’. Because this mainly contains core code of the system, which is not likely to contain serious faults, a Blue Screen referring to ‘Page Fault in Non-Paged area’ probably indicates a serious hardware problem with the RAM modules, or possibly damaged code resulting from a defective Hard disk. It is, though, possible that external utility software (e.g. Norton) may put modules there too, so if such faults arise when you have recently installed or updated something of this sort, try uninstalling it.

• The Page Pool which can be used to hold:
  • Program code,
  • Data pages that have had actual data written to them, and
  • A basic amount of space for the file cache (known in Windows 9x systems as Vcache) of files that have recently been read from or written to hard disk.

Any remaining RAM will be used to make the file cache larger.

Why is there so little Free RAM?

Windows will always try to find some use for all of RAM — even a trivial one. If nothing else it will retain code of programs in RAM after they exit, in case they are needed again. Anything left over will be used to cache further files — just in case they are needed. But these uses will be dropped instantly should some other use come along. Thus there should rarely be any significant amount of RAM ‘free’. That term is a misnomer — it ought to be ‘RAM for which Windows can currently find no possible use’. The adage is: ‘Free RAM is wasted RAM’. Programs that purport to ‘manage’ or ‘free up’ RAM are pandering to a delusion that only such ‘Free’ RAM is available for fresh uses. That is not true, and these programs often result in reduced performance and may result in run-away growth of the page file.

Where is the page file?

The page file in XP is a hidden file called pagefile.sys. It is regenerated at each boot — there is no need to include it in a backup. To see it you need to have Folder Options | View set to ‘Show Hidden and System files’, and not to ‘Hide Protected mode System files’.

In earlier NT systems it was usual to have such a file on each hard drive partition, if there were more than one partition, with the idea of having the file as near as possible to the ‘action’ on the disk. In XP the optimisation implied by this has been found not to justify the overhead, and normally there is only a single page file in the first instance.

Where do I set the placing and size of the page file?

At Control Panel | System | Advanced, click Settings in the “Performance” Section. On the Advanced page of the result, the current total physical size of all page files that may be in existence is shown. Click Change to make settings for the Virtual memory operation. Here you can select any drive partition and set either ‘Custom’; ‘System Managed’ or ‘No page file’; then always click Set before going on to the next partition.

Should the file be left on Drive C:?

The slowest aspect of getting at a file on a hard disk is in head movement (‘seeking’). If you have only one physical drive then the file is best left where the heads are most likely to be, so where most activity is going on — on drive C:. If you have a second physical drive, it is in principle better to put the file there, because it is then less likely that the heads will have moved away from it. If, though, you have a modern large size of RAM, actual traffic on the file is likely to be low, even if programs are rolled out to it, inactive, so the point becomes an academic one. If you do put the file elsewhere, you should leave a small amount on C: — an initial size of 2MB with a Maximum of 50 is suitable — so it can be used in emergency. Without this, the system is inclined to ignore the settings and either have no page file at all (and complain) or make a very large one indeed on C:

In relocating the page file, it must be on a ‘basic’ drive. Windows XP appears not to be willing to accept page files on ‘dynamic’ drives.

NOTE: If you are debugging crashes and wish the error reporting to make a kernel or full dump, then you will need an initial size set on C: of either 200 MB (for a kernel dump) or the size of RAM (for a full memory dump). We Recommend a Full System Scan Download Here

Can the Virtual Memory be turned off on a really large machine?

Strictly speaking Virtual Memory is always in operation and cannot be “turned off.” What is meant by such wording is “set the system to use no page file space at all.”

Doing this would waste a lot of the RAM. The reason is that when programs ask for an allocation of Virtual memory space, they may ask for a great deal more than they ever actually bring into use — the total may easily run to hundreds of megabytes. These addresses have to be assigned to somewhere by the system. If there is a page file available, the system can assign them to it — if there is not, they have to be assigned to RAM, locking it out from any actual use.

How big should the page file be?

There is a great deal of myth surrounding this question. Two big fallacies are:

• The file should be a fixed size so that it does not get fragmented, with minimum and maximum set the same
• The file should be 2.5 times the size of RAM (or some other multiple

Problems with Virtual Memory

It may sometimes happen that the system give ‘out of memory’ messages on trying to load a program, or give a message about Virtual memory space being low. Possible causes of this are:

• The setting for Maximum Size of the page file is too low, or there is not enough disk space free to expand it to that size.



• The page file has become corrupt, possibly at a bad shutdown. In the Virtual Memory settings, set to “No page file,” then exit System Properties, shut down the machine, and reboot. Delete PAGEFILE.SYS (on each drive, if more than just C:), set the page file up again and reboot to bring it into use.



• The page file has been put on a different drive without leaving a minimal amount on C:.



• There is trouble with third party software. In particular, if the message happens at shutdown, suspect a problem with Symantec’s Norton Live update, for which there is a fix posted here. It is also reported that spurious messages can arise if NAV 2004 is installed. If the problem happens at boot and the machine has an Intel chipset, the message may be caused by an early version (before version 2.1) of Intel’s “Application Accelerator.” Uninstall this and then get an up-to-date version from Intel’s site.



• Another problem involving Norton Antivirus was recently discovered by MS-MVP Ron Martell. However, it only applies to computers where the pagefile has been manually resized to larger than the default setting of 1.5 times RAM — a practice we discourage. On such machines, NAV 2004 and Norton Antivirus Corporate 9.0 can cause your computer to revert to the default settings on the next reboot, rather than retain your manually configured settings. (Though this is probably an improvement on memory management, it can be maddening if you don’t know why it is happening.) Symantec has published separate repair instructions for computers with NAV 2004 and NAV Corporate 9.0 installed. [Added by JAE 2/21/06.]



• Possibly there is trouble with the drivers for IDE hard disks; in Device Manager, remove the IDE ATA/ATAPI controllers (main controller) and reboot for Plug and Play to start over.



• With an NTFS file system, the permissions for the page file’s drive’s root directory must give “Full Control” to SYSTEM. If not, there is likely to be a message at boot that the system is “unable to create a page file.”

Its Highly recommeded that you perform a full system scan if experiencing Virtual Memory Stop Errors. This can lead to potential system crash and loss of data. Click on scan now buttom to begin scanning system.