Re: Etherboot 5.5

Michael Brown wrote:
> On Thu, 19 May 2005, Timothy Legge wrote:
> 
>>>  The ability to operate completely in 16-bit real mode, never
>>>  switching to protected mode.  This is necessary in order to comply
>>>  fully with the PXE "specification".  This is controlled by a
>>>  compile-time option -DKEEP_IT_REAL; off by default.
>>
>>I understand that this is one of the more difficult parts of the PXE
>>spec.  Doesn't it contradict some other requirement in the spec (not
>>that that s unusual?
> 
> 
> Operating in real mode is not the really tricky part; that would be
> operating in 16-bit protected mode (which we're supposed to do with the
> same binary code).  There are a few interesting theoretical
> impossibilities that the spec throws at us, such as requiring us to write
> to specified arbitrary physical addresses when we're in 16-bit pmode and
> have enforced 64kB segment limits.
> 

... without access to the GDT.  Of course, if you're in CPL0 you can 
always hijack the GDT, at least as long as you turn interrupts off.

Real-life hardware seems to have "solved" this by hiding an back door 
somewhere on the card, either in I/O space or in PCI Config space.

	-hpa



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