Windows 2000 Advanced Server and Windows 2000 Datacenter
Server support memory in excess of 4 gigabytes (GB) of random access memory (RAM) by way of the Intel
Physical Addressing Extension (PAE) specification. Windows 2000 Advanced Server
is limited to 8 GB, and Windows 2000 Datacenter Server is limited to 32 GB. You
can enable PAE in the Boot.ini file. To enable PAE, follow these steps:
- Click Start, and then click Run.
- Type X:\boot.ini, where
X is the drive letter of the location of the boot
files, Ntldr, Boot.ini, and so forth.
- Modify the line that corresponds to your operating system
by appending the /PAE switch.
- Save the file, and then restart the computer.
Even with PAE enabled, the underlying architecture of the
system is still based on 32-bit linear addresses. This effectively retains the
2 GB of application space and the 2 GB of kernel mode space because only 4 GB
of addresses are available. However, multiple processes can immediately benefit
from the increased RAM because they are less likely to encounter physical
memory restrictions and begin paging. Additionally, applications can be
modified to use the AWE API to allocate memory outside of the applications
process space, bypassing the 2-GB limit for applications.
With PAE enabled, the operating system moves from a
two-level linear address translation to a three-level address translation. The
extra layer of translation is what provides access to physical memory beyond 4
GB. Instead of a linear address being split into three separate fields for
indexing into memory tables, it is split into four separate fields; a 2-bit
field, two 9-bit fields, and a 12-bit field that corresponds to the page size
implemented by Intel Architecture (4 KB).
During a context switch
the CR3 register is set by the operating system to point to a Page directory
pointer index that is 2-bits wide. The first two bits are used as an index into
this table, with the resulting value pointing to a Page directory. The first
9-bit field is then used to index into the Page directory. The indexed value
then points to a Page table. The second 9-bit field is an index into the Page
table. This value points to the actual page in memory where the desired byte is
located. Finding this byte is a simple matter of using the remaining twelve
bits of data to index into the page.
If you add more memory, the BIOS may recognize the full amount of physical RAM that is installed on the server. However, Windows will recognize only some of the RAM. If the server includes the Redundant Memory feature, the full amount of RAM is not exposed to Windows. The Redundant Memory feature provides the system with a fail-over memory bank when a memory bank fails. The Memory Mirroring feature splits the memory banks into a mirrored set. Both features are enabled in the BIOS and cannot be accessed through Windows. You may have to see the system user manual or the manufacturer's Web site to modify the settings for this feature.
For example, if you are running a system that has 4 GB of RAM installed, and you add 4 GB of RAM, Windows recognizes 4 GB or 6 GB of RAM instead of the full 8 GB of physical memory. One of these features may be enabled on the new memory banks without your knowledge. In this scenario, the symptoms are similar to the symptoms that may occur when the /PAE switch is not added to the Boot.ini file.