[SeaBIOS] Some questions
kevin at koconnor.net
Thu Nov 25 14:35:06 CET 2010
I'm CC'ing the mailing list.
On Wed, Nov 24, 2010 at 11:45:17AM -0500, Stefan Berger wrote:
> Hi Kevin,
> while I am porting the BIOS code for TCG support to SeaBIOS, I am
> wondering about the following:
> 'When' is it recommendable to switch from 16 bit to 32bit.
To be honest, this has never really been done before in SeaBIOS, so I
think we'll have to figure this out as we go. Since you need to
access full memory, it looks like it will be required to jump into
> asking because the interface I am implementing allows callers to
> pass input and output data structures using es:di and ds:si, with es
> and ds allowed to be loaded with anything possible. So my strategy
> in the Bochs BIOS was to switch to 32 bit even before calling the
> interrupt handler, there doing the case statement for branching into
> the actual functions (switch (regs->al)) and passing 32 bit pointers
> converted from es:di and ds:si to the functions doing the work and
> reading from those memory locations.
This sounds okay to me - it should be possible to implement
handle_1abb(regs) which does a call32(tcg32_1abb, regs). It's
possible to do all the pointer conversion while in 32bit mode.
>The 'thing' is also that
> several ones of the functions that can be called through the
> interrupt interface are also called from the implemented functions.
> Functions A,B,C can be called via the interrupt handler.
> Function B also calls A.
> Function C also calls B.
This shouldn't really be an issue. The build should take care of
> This is due to the hierarchical nature of the functions. Also, while
> for example B calls A, B fills a data structure (on the stack) that
> it passes to A. A of course can get the same data structure from the
> user calling the interrupt interface. So, by switching to 32 bit
> mode and converting to 32 bit pointers early I could previously
> avoid a lot of headaches with the segment registers in 16 bit mode
> and reading the data from wherever they may be. Would you have any
> concerns about switching to 32 bit mode early, so that the interrupt
> handler and anything subsequent runs in 32 bit mode?
I can't say for sure what will make sense without seeing the code
first, but it sounds okay to me.
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