[Bug 217512] boot2 is unable to load kernel directly

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217512

--- Comment #2 from Mark Willson <mark at uncommon.com> ---
As I understand the theory of  boot0/boot1/boot2/loader operations, boot2
should have all the registers, bios calls, and setup correct for direct
execution of the kernel. Assuming, of course, that the kernel has everything
that it needs compiled into the kernel [with the exception of kernel modules]
and does not need any of the modifications or the boot time services, e.g.
device.hints, loader.conf, etc., that the loader would supply.

Assuming there is no support for AMD64s in boot2 then--

1) there is no support for any 64 bit based CPUs (intel or amd), effectively
limiting its use for booting directly into the kernel to ancient 32-bit i386
machines

2) boot.config, for the same purposes, will not work

3) boot2 cannot be used to bypass loader problems on 64 bit machines

4) boot2 can only be used to look at or changing certain temporary variables,
select a different loader (very unlikely), or to fiddle with video/serial
console options

5) the documentation is in error in a number of places (e.g. handbook, man,
internals) and needs to note this major limitation so that others don't go down
rabbit hole as well. 

I did do some additional checking and it appears that you are correct at least
as far back as FreeBSD 8.x. A similar error/outcome came up on a different
server/bios/config.

I also noted that boot2 does say 'FreeBSD/i386 BOOT' at its prompt (although,
since i386/amd64 are the core lines for freebsd I've always thought of i386 as
a subset of AMD64--anything i386 could do amd64 could do better--guess I was
wrong).

Is it possible that AMD64s *could* work if the appropriate device.hints were
statically linked in the kernel itself as well as any other "external" required
setup done by the loader at boot time is compiled in or is there something
fundamental about the architecture/BIOS support that makes this unlikely?

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