What is the A20 line?
The A20 line takes a bit of explaining so hang with me please. When the AT was introduced, it was able to access up to sixteen megabytes of memory, but in order to remain compatible with the IBM-XT, due to a quirk in the XT architecture (memory wraparound) had to be duplicated in the AT so things would work the same, the 20th address line on the bus (A20) was turned off so this “wrap-around” effect worked and software from the old XT days continued to work (compatibility was a *B*I*G* issue back then). The A20 line is controlled by the keyboard controller unit. Which is usually a derivative of the 8042 chip.
