Re: [Xen-devel] Re: define BOOT_TRAMPOLINE and stack based on result of probing EBDA area by INT12

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 31/08/2011 20:25, "Alan Cox" <alan@xxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, 31 Aug 2011 09:55:10 +0100
> Keir Fraser <keir@xxxxxxx> wrote:
> 
>> On 31/08/2011 09:47, "Lin-bao Zhang" <zhang.linbao@xxxxxxxxx> wrote:
>> 
>>> 1,define a variable named "EBDA_bottom".
>>> 2, get EBDA_bottom by above method.
>>> 3, stack should equals EBDA_bottom (or EBDA_bottom -1 safely)
>>> 4, mov     $(EBDA_bottom -1),%esp
>>> in most case , EBDA area is 1K,but we define 0x7c000(this is absolutely
>>> safe),but we will waste too much memory space.
>>> 
>>> I did test, it can work .Certainly, I am familiar with assembler code, I
>>> just
>>> hard code to test:mov     0x903ff , %esp thanks for your corrections , I
>>> have
>>> not read over all histories and stories about them, if I am wrong , I am
>>> sorry
>>> first.
>> 
>> If you actually tried to implement it you'd realise you're stuck.
> 
> Re-read the original. The EBDA is accessible at BIOS segment offset 0E.
> You don't need to make a BIOS call to read it, just load the location and
> check it against 0.W in which case one isn't present.
> 
> At that point you know where to put your bits.
> 
> Obviously once you get into the world of EFI and the like there are
> different ways all this should occur, but for good old BIOS stuff it
> works fine.

Ah, makes sense. And our real-mode code is now relocatable, which was
implemented as part of support for EFI. That could be used to dynamically
relocate below EBDA for legacy BIOS too.

 -- Keir



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