A20 Gating


IBM AT, and early clones of

The BIOS controls A20 behaviour via the motherboard's keyboard controller chip, and a 74257 chip.

An IBM AT (IBM 5170) diagram is at here


Some later clones

A20 gating implemented as part of a chipset.

The chipset does the switching of A20 behaviour faster than what the motherboard's keyboard controller chip can do.

Some BIOS' allow you to enable/disable the chipset from controlling A20 behaviour.  It may be referred to in CMOS SETUP as something like 'FAST A20'.  Presumably, when disabled, A20 behaviour goes back to being controlled by the motherboard's keyboard controller chip.

For example, the Chips and Technologies 82C351 chip (part of the CS82310 chipset) contains 'GATE A20' logic.

CS82310 chipset - A20.png


Some later clones: 80486 based, and later

See the 'A20 Gate' section of here, specifically the paragraph starting, "The Intel 80486 and Pentium added a special pin named A20M#", and later paragraphs.