Microsoft Knowledge Base Article - 183063

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager \Memory Management\ SecondLevelDataCache

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.

I will translate.
If the value of the Secondlevelcache is zero.
The system tries to access this registry key as though it were larger than zero first.
If the system finds that the registry key is zero, it will use that size for secondleveldatacache.

The zero used in this registry key is really 256 Bytes.

Now for the scarry part.

Microsoft makes this registry key zero (256 Bytes), on ALL nt kernel operating systems.
And in doing so limits the processor secondleveldatcache to 256 Bytes.

To use a secondleveldatacache larger than 256 Bytes.
You have to edit the nessessary registry key.

I have 512 Bytes secondleveldatacache.
So I put Decimal 512, Hexidecimal 200. Instead of zero, in that registry key.

If you have a pentium 4 extreme edition processor with 2 megaBytes secondleveldatacache.
You put Decimal 2048, Hexidecimal 800, instead of zero in this registry key.

Here is a calculator to see for yourself.
http://www.speedguide.net/conversion.php

To get to this registry key.
First press the start button.
Then press the run button.
Then type regedit and press ok.
Now go and follow this path
HKEY_LOCAL_MACHINE\
SYSTEM\
CurrentControlSet\
Control\
Session Manager \
Memory Management\
SecondLevelDataCache

I'll summarize with a quote from Microsoft Knowledge Base Article - 183063
"Setting SecondLevelDataCache to 256 KB(0) rather than 2 MB (when the computer has a 2 MB L2 cache) would probably have about a 0.4 (40%)percent performance penalty."

What this registry key is set at has real performance results.