A20 gate

On the x86 platform, another vestige of the legacy 1 MB address limitation is usually found in something called an A20 gate. This is a piece of hardware that would force the A20 address line to zero, regardless of the actual setting of the A20 address line on the processor.

The justification for this was for legacy software that would depend on the ability to wrap past location 0xFFFFF back to 0x00000. The QNX Neutrino RTOS doesn't have such a requirement. As a result, the OS doesn't need any A20 gate hardware to be installed. Note that some embedded x86 processors have the A20 gate hardware built right into the processor chip itself—the IPL will disable the A20 gate as soon as possible after startup.

Note: If your system requires a standard BIOS, there's a small chance that the BIOS will make use of the A20 gate. To find out for certain, consult your BIOS supplier.