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Enable Fast A20 Gating
The A20 line is the 21st address line, and is used to control access to the high memory area, which is the first 64 KB of extended memory. This subject is confusing, as the preceding sentence no doubt illustrates :^) but I have covered it in some detail in this section of the Reference Guide. The A20 line was historically managed by the keyboard controller, however newer motherboards allow it to be controlled directly by the chipset. This feature is usually called "Fast A20" or "A20 Gating", and is enabled with a BIOS setting. This setting should be enabled for a small performance boost.
Note: It is unusual in my
experience for problems to result from using this setting, so if you are having trouble
enabling this you may want to troubleshoot
the keyboard controller.