On Saturday 25 October 2008 04:09:45 am Jeremy Chadwick wrote:
> Just for posterity: the USB Legacy Support BIOS option does not affect
> natively-connected PS/2 keyboards; you can leave the option enabled even
> in the scenario where you have a USB keyboard *and* a PS/2 keyboard
> plugged in; one will not "trump" the other. Instead, you should have
> two keyboards which function in OSes/environments which lack a USB
> stack. (That is, until something resets/reassigns the BIOS-controlled
> interrupt, which will then break USB->PS/2 emulation; the native PS/2
> keyboard should not be affected by this)


This last statement is not quite true (at least not always true). For many
systems, the way the PS/2 emulation works is that accesses to the backing I/O
ports (0x60 and 0x64) case a trap into SMM and the SMI handler in the BIOS
then talks to the USB controller and keyboard and updates the register values
to simulate the I/O port accesses. This is disabled by having the USB host
controller driver frob flags in controller registers to disable the SMI
traps.

--
John Baldwin
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