Tyan S2460 and btx.S

[Walter C. Pelissero]

Apparently FreeBSD 5.x is neither able to power off nor reset a Tyan
Tiger MP (S2460) motherboard.  I believe it's a known problem.  While
the power off may be due to a broken ACPI the reset can be fixed.

Revision 1.33 log of /sys/boot/i386/btx/btx/btx.S says:

  date: 2003/11/16 18:24:23;  author: phk;  state: Exp;  lines: +1 -1
  When rebooting the machine jump to 0xf000:0xfff0 instead of 0xffff:0x0.

  While we end up the same place, we end up with two different CS register
  values after the jump and 0xf000 is compatible with the hardware reset
  value.

  This makes a difference if the BIOS does a near jump before a far jump.

  Detective work and patch by:	 Adrian Steinmann <ast at marabu.ch>

I tried reverting the patch and now the Tyan motherboard seems to
reset as expected.

My knowledge of the PC architecture intricacies is not good enough to
discuss the merits of revision 1.33 patch, but I believe it could be
worth having an ifdef-ed code in btx.S to accommodate motherboards
like this one.

-- 
walter pelissero
http://www.pelissero.de