Re: [Etherboot-developers] Q: does xstart work?

[Eric W B. <ebi...@ln...>]

ke...@us... (Ken Yap) writes:

> >Staring at the assembly in xstart there is the following code
> >snippet:
> >	pushl	%eax
> >	ADDR32	LJMPI(_execaddr-_start16)
> >1:
> >...
> >_execaddr:
> >	.long	0
> >
> >What I don't get is how this snippet of code successfully jumps
> >to an application.  ADDR32 should ljmpi should take a m16:32.
> >Where the offset which comes first is 32 bits, and the segment
> >that comes after is 16bits.  I just don't see how that works when
> >we pass it a m16:16 in execaddr.
> 
> _execaddr is filled in with a linear address.

For 32 bit code yes.  But for 16bit code it is clearly a filled with
a segment offset to jump to.
From the nbi spec:

          |                     |
          | Initial Magic No.   |  4 bytes
          +---------------------+
          |                     |
          | Flags and length    |  double word
          +---------------------+
          |                     |
          | Location Address    |  double word in ds:bx format
          +---------------------+
          |                     |
          | Execute Address     |  double word in cs:ip format 
          +---------------------+
  ______________________________________________________________________

And Execute Address is the ultimate source of the address.

Eric