Re: [NILO-discuss] Zen and the art of running in real mode

[Michael B. <mb...@fe...>]

On Wed, 9 Mar 2005, H. Peter Anvin wrote:
> >>Right.  With appropriate descriptors set up they could work in 16-bit 
> >>protected mode, though, for supporting the PM entry point.
> > They could, but that wouldn't help the code when it's called via the RM 
> > entry point.
> Supporting V86 mode entry via the RM entry point is probably futile.

If we support "pure real mode" operation, then it should just work in V86 
mode.

> > Also, the cards we're talking about are quite likely to need more than 
> > 64kB of data buffers, in which case we can't fit it into the strict UNDI 
> > model (64kB code + 64kB data+bss+stack, all in base memory) anyway.  We 
> > have to accept that the PXE specification is actually incompatible with 
> > some network cards, and plan to carefully violate the spec.
> Indeed, although the !PXE structure clearly is designed to accommodate 
> additional descriptors.

But only two of these can simultaneously satisfy the requirements of being
located in base memory and being used by the UNDI portion, as I've pointed
out before.

> (Note: for a lot of cards which *can* have > 64K data buffers, generally 
> don't *have* to.  For example, a device with a 1024-entry descriptor 
> ring can be filled with only a few entries, on the assumption that 
> performance is secondary in our application.)

The most recent gigabit driver I wrote ended up dropping packets (and so 
waiting for TFTP retransmits) with fewer than 66kB of buffers.  Though 
it's true that we're not after high performance, having the occasional 
multi-second pause is not acceptable.

Michael