The A20 gate story was something different though, it abused the numeric overflow of 16 bit addresses, not some unused bits in the middle of an address. An x86

The A20 gate story was something different though, it abused the numeric overflow of 16 bit addresses, not some unused bits in the middle of an address. An x86 adres had a 16 bit segment and a 16 bit offset, with the address being calculated as 16*segment+offset, truncated to 20 bits. Segment F000 was in ROM and 0000 was in RAM, with the low RAM addresses used for different BIOS functionalities. If you use a segment like 0xFF00 , then offset 0 to 0x0FFF correspond to linear addresses 0xFF000 to 0xFFFFF, in ROM. Offset 0x1000 to 0xFFFF correspond to 0x0000 to 0xEFFF, in RAM. This trick meant you can set the segment register only once, and use the offset to read both tables in ROM and use variables in RAM. Of course contemporary BIOS and other software used this trick to shave of a few instructions. You know what happens next. The 80286 has 24 address lines, not 20, so the addresses just above the existing 0xFFFFF = 1MB became valid instead of wrapping around. In the name of backward compatibility, someone found an unused port on the keyboard controller, attached an AND port to it and to A20. If you talk to the keyboard controller, you can choose if you want 16MB ram or the old wraparound behavior. The keyboard controller was of course dog slow, and you have to set and reset the A20 gate the whole time to switch between not triggering BIOS/DOS/TSR bugs and usable upper RAM. This hack is AFAIK still there today in every x86 based PC, even if the gate stays enabled all the time.