[syslinux] Chainloading DRMK

[Gert Hulselmans]

Gene Cumm wrote:
> DRMK (Dell Real Mode Kernel), now used in their DOS-based diagnostic
> images (probably about 2-4 years), has a rather unusual entry into the
> IO file (DELLBIO.BIN; similar to IO.SYS and IBMBIO.COM).  After
> fumbling around with it over the last few days, I _think_ I've figured
> out the quirks of this entry point (and successfully tweaked chain.c32
> to do such).
>
> dl Usual; drive number
> bp 0
>
> cs 0x0070
> ss 0
> ds Location of relocated boot sector code that loaded it; only used
> for 1 data element then initialized.
>
> The following registers are initialized by DELLBIO.BIN prior to use:
> ax, cx, si, di, sp, and es.
>
> fs and gs were 0 but are not used by DELLBIO.BIN.
>
> I believe bx is a remnant from loading DELLBIO.BIN and probably
> initialized by DELLBIO.BIN prior to use.  On entry it is equal to the
> file size & 0xFC00.
>
> In addition to those register values, it also must have a value of
> 0x0000003f (little endian, of course) at ds:[bp+28].  I still need to
> trace around to find what that's used for.
>
> Gert Hulselmans, thank you for your help and making others aware that
> it couldn't be chainloaded from Syslinux.
>
> --
> -Gene
>
Just for the people who wonder about the idea of chainloading DRMK (Dell
Real Mode Kernel). It started at this topic:
http://www.boot-land.net/forums/index.php?showtopic=12052&st=10

@ Gene Cumm
Thanks for spending some time on it to figure it all out.

-Gert Hulselmans