The AT keyboard adds a keyboard controller.
The numeric keypad is now separated from the main keyboard.
There is a single new key, with scancode 84 = 54,
namely SysRq.
The protocol for AT and later keyboards differs from that for
XT keyboards. Some old keyboards have an XT/AT switch on the
backside that selects the appropriate protocol.
Other keyboard autodetect XT or AT mode.
The KeyTronic KB101-1 keyboard has four switches of which the first two
indicate the desired behaviour (00 - autodetect, 01 - unused,
10 - PC/XT, 11 - AT). Autodetect does not always work.
