With a 16-bit operand, lgdt/lidt will load only a 24-bit base address,
ignoring the high-order bits.  This meant that we could fail to fully
restore the GDT across a call into gPXE, if the GDT happened to be
located above the 16MB mark.

Not all of our lgdt/lidt instructions require a data32 prefix (for
example, reloading the real-mode IDT can never require a 32-bit base
address), but by adding them everywhere we will hopefully not forget
the necessary ones in future.

See http://etherboot.org/wiki/dev/gdbstub for documentation.

confusing to read the code.

stack; it breaks vmxassist.

outside of librm.S.

real_call(), rather than moving it to the RM stack and back again.
This allows the real-mode function to completely destroy the stack
contents, provided that it manages to return to real_call().

call.

worrying about its stack being destroyed during the Etherboot run.

install() now calls relocate(), moves the protected-mode code to the new
location, and calls hide_etherboot().

Use .text16.data section with "aw" attributes, to avoid section type
conflicts when placing both code and data into .text16.

Add __from_{text16,data16}.

between the low half stored in the static variable rm_sp, and the high
half stored on the prot_call() stack, because:

  Just using the stack would screw up when a prot_call()ed routine
  executes a real_call(); it would have no way to find the current top of
  the RM stack.

  Extending rm_sp to rm_esp would not be safe, because the guarantee that
  rm_sp must return to the correct value by the time an external
  real-mode call returns applies only to %sp, not to %esp.

from protected-mode code.

Set up %ds to point to .data16 in prot_to_real, so that code specified
via REAL_EXEC() and friends can access variables in .data16.

Move most real-mode librm variables from .text16 to .data16.

install_prealloc.  I *think* these will suffice for all the 16-bit
prefixes.

advantage of the fact that we have to have a permanently-resident block
in base memory.

Doing so, in fact, seems to expose an assembler bug; (a-b-0) is
apparently not the same as (a-b).  Go figure.

of librm.

always saves 32 bits).

registers returned to RM code.