Apparently, DOS and possibly other legacy operating systems issued a
null command to the keyboard controller after toggling A20,
specifically "pulse output pins" with no output pins specified. This
was presumably done for synchronization reasons. This has made it
into at least the UHCI spec, and it has been found to cause
compatibility problems when "legacy USB" is enabled (which it almost
always is) to not have this byte sent.
It is *NOT* clear if any of these compatibility problems has any
effect on Linux. However, for maximum compatibility, issue this null
command after togging A20 through the KBC.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Remove old comments that include the old arch/i386 directory.
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
A20 handling code for the new x86 setup code. This implements the same
algorithms as the assembly version.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
H. Peter Anvin