Use of Memory Configurations Greater than 512MB in Windows 95, 98 and Windows Millennium Edition

There have been numerous questions that have arisen since the release of Windows 95 regarding memory limitations in Windows 9x. These questions resurface with each subsequent release, and cover Windows 95 OSR2, Windows 98, Windows 98SE and the Windows Millennium Edition. Windows 9x does have a memory limitation, however if your needs require in excess of 512MB of physical memory, there are methods you can use to overcome this limitation.

The information in this document is supported by the following articles that can be found in the Microsoft® Product Support Services Technical Knowledge Base:

Out of Memory Error Messages with Large Amounts of RAM Installed

Some symptoms to help you identify if your are experiencing this problem

If a computer that is running any of the versions of Windows listed above contains more than 512 megabytes (for example, 768 megabytes) of physical memory (RAM) installed, you may experience one or more of the following symptoms:

The Cause
The Windows 32-bit protected-mode cache driver (Vcache) determines the maximum cache size based on the amount of RAM that is present when Windows starts. Vcache then reserves enough memory addresses to permit it to access a cache of the maximum size so that it can increase the cache to that size if needed. These addresses are allocated in a range of virtual addresses from 0xC0000000 through 0xFFFFFFFF (3 to 4 gigabytes) known as the system arena.

In Computers having large amounts of physical RAM memory, the maximum cache size can be large enough that Vcache consumes all of the addresses in the system arena, leaving no virtual memory addresses available for other functions such as opening an MS-DOS prompt (creating a new virtual machine).

Possible Workarounds
Try one of the following methods:

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article.

Vcache is limited internally to a maximum cache size of 800 MB.

This problem may occur more readily with Advanced Graphics Port (AGP) video adapters because the AGP aperture is also mapped to addresses in the system arena. For example, if Vcache is using a maximum cache size of 800 MB and an AGP video adapter has a 128-MB aperture mapped, there is very little address space remaining for the other system code and data that must occupy this range of virtual addresses.

This page updated: 11/01/2000

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