Microsoft Knowledge Base Article - 99588 , 183063

This topic discusses the Registry key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager \Memory Management\SecondLevelDataCache

If this registry key is set as discussed to zero by hal. The amount of level 2 cache is restricted to 256 Bytes.
The amount of level 2 cache set in this registry IS the amount used.
I have 512 Bytes level 2 cache. So I set the registry key to Decimal 512, Hexidecimal 200.
Here's a link to look at the setting.
http://www.lockergnome.com/issues/daily/20000529.html

Microsoft Knowledge Base Article - 99588

The Windows NT hardware abstraction layer (HAL) refers to a layer of software that deals directly with your computer hardware.
Because the HAL operates at a level between the hardware and the Windows NT executive services,
applications and device drivers need not be aware of any hardware-specific information.
The HAL provides routines that enable a single device driver to support a device on different hardware platforms,
making device driver development much easier. It hides hardware dependent details such as I/O interfaces, interrupt controllers,
and multiprocessor communication mechanisms.
Applications and device drivers are no longer allowed to deal with hardware directly and must make calls to HAL
routines to determine hardware specific information. T
Thus, through the filter provided by the HAL, different hardware configurations can be accessed in the same manner.

Microsoft Knowledge Base Article - 183063

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your
operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly.
Use Registry Editor at your own risk.

SecondLevelDataCache records the size of the processor cache, also known as the secondary or L2 cache.
If the value of this entry is 0, the system attempts to retrieve the L2 cache size from the Hardware Abstraction Layer (HAL) for the platform.
If it fails, it uses a default L2 cache size of 256 KB. If the value of this entry is not 0, it uses this value as the L2 cache size.
This entry is designed as a secondary source of cache size information for computers on which the HAL cannot detect the L2 cache.

This is not related to the hardware; it is only useful for computers with direct-mapped L2 caches.
Pentium II and later processors do not have direct- mapped L2 caches.
SecondLevelDataCache can increase performance by approximately 2 percent in certain cases for older computers
with ample memory (more than 64 MB) by scattering physical pages better in the address space so there are not so many L2 cache collisions.
Setting SecondLevelDataCache to 256 KB rather than 2 MB (when the computer has a 2 MB L2 cache) would probably have
about a 0.4 percent performance penalty.

Translation. If hal does not change this setting to use the full level 2 cache available.
The performance can suffer up to 40 percent. According to Microsoft.