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technically be necessary, because the "enable A20" command requires only that the keyboard controller is ready to accept input (i.e. that its input buffer is empty), and shouldn't also require that the keyboard is ready to send output (i.e. that its output buffer is also empty). See http://www.smsc.com/main/tools/io-bios/42i.pdf section 3.1 ("Command Invocation") for a justification. gateA20_set() is called on every real-mode transition (in case some idiot piece of external code such as Intel's PXE stack decided it would be fun to re-disable A20), so draining the keyboard buffer means that we end up losing keypresses on some systems. In particular, this makes typing at the command line almost impossible, and causes Etherboot to ignore Ctrl-Alt-Del. We should really implement a gateA20_test() function to verify that gate A20 has been correctly enabled, and think about adding other commonly-used methods such as Fast Gate A20. |
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