[a20] Send a null command to the KBC after changing A20
authorH. Peter Anvin <hpa@zytor.com>
Fri, 27 Jun 2008 21:29:13 +0000 (22:29 +0100)
committerMichael Brown <mcb30@etherboot.org>
Fri, 27 Jun 2008 21:29:13 +0000 (22:29 +0100)
commitb7882065f5319d8d8f93eb4b879d816bb47bfe3a
tree8ec8ab67ff0baf31c1580b0c7abbca5b78695042
parent2b9e7a4767b73cfe10324fbc2fed07646ee4a33c
[a20] Send a null command to the KBC after changing A20

Send a null command, specifically "pulse outputs" with no outputs
selected, to the KBC after changing A20.  This was apparently done by DOS,
presumably as a synchronization hack, and the authors of the UHCI spec
thought it was inherent.  Therefore, there are systems out there (e.g. HP
DL360 G5) which will stop responsing to "legacy USB" unless they see the
null command, 0xFF, written to port 0x64 at the end of the A20 toggling
sequence.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
src/arch/i386/firmware/pcbios/gateA20.c