BTX loader reboot on Soekris comBIOS1.22 fails (patches for btx.s and loader/main.c enclosed)

[Adrian Steinmann]

Soren

We seem to have localized the loader reboot hang back to the first
far jump happening in btx.s:

<ast at marabu.ch> observed:
   > warm boot (writes 0x1234 to 0x472) and then jumps to the BIOS reboot
   > handler:
   > -		ljmp $0xffff,$0x0		# reboot the machine
   >
   > however in various literature it is mentioned that $0xf000,$0xfff0
   > is bound to work better on most platforms, so I tried
   > +		ljmp $0xf000,$0xfff0		# reboot the machine
   >
   > which indeed works! (OpenBSD, for example, uses ljmp $0xf000,$0xfff0).

<soren at soekris.com> replied:
   The reason is that on some hardware (t.ex the Geode) there need to be
   work done early on in the BIOS to enable access to the BIOS in low
   memory before that first far jump, and therefore the first jump is not a
   far jump as on the original PC, but a near jump. Having the segment set
   at FFFF can screw up that first near jump....

This is exactly what I have read in the book "The Undocumented PC" by
Frank van Gilluwe,  1st ed., Addison Wesley 1994, ISBN 0-201-62277-7,
p91ff. In there, he also says that using INT19 or jumping to BIOS boot
f000:ffff are "misconceptions" and that software control (keyboard reset)
should be used whenever possible (hence both of my patches). I will wait
for feedback from some Cc: people and if required, submit PRs to
fix btx.s and loader/main.c.

<soren at soekris.com> then opined:
   So the ljmp $0xf000,$0xfff0 would be the best way, but since my BIOS
   patches F000:FFF0 after copying the BIOS from flash to ram, I could also
   change my near jump to a far jump to increase compatibility....

Both! FreeBSD should fix the jmp in btx.s and you could do an initial
far jump for increased compatibility.

Adrian