[OpenBIOS] Summary: Booting the OS

[Alle Metzlar]

Jim Stewart wrote:

> Chris Arguin wrote:
> >
>
> > Off-the-wall idea #132: How much legacy BIOS support do we really need
> > *just* to boot DOS to the point that it loads config.sys and/or
> > autoexec.bat? If we supported just that much, and then had a program you
> > ran (first) in DOS that basically put a legacy BIOS in as if the real
> > BIOS had been shadowed, then we have the best of both worlds. A great BIOS
> > with that supports all the nice boot-stuff w/o too much legacy code, but
> > available legacy code when you need it for DOS.
>
> I can answer that,
>
>         Int 10h, function Eh            teletype ouput          mostly trivial for serial port
> redirect, done for you with most vga cards
>         Int 11h                         get equipment config    trivial
>         Int 12h                         mem size                trivial
>         Int 13h  function 0,1,2         disk read write reset   not trivial, probably
> don't need write for boot, some dos need funct 8
>         Int 16h  function 0,1           keyboard poll and read  mostly trivial for serial
> port redirect, don't know about PC keyboard
>         Int 1Ah  function 0,1,2,3       RTC                     ok if only stub code to return a valid
> constant - must not return nonsense
>         Int 19h                         Bootstrap               trivial
>

Why any legacy BIOS support? The answer to this question, in my opinion, highly depends on the hardware
and operating systems that are going to be supported.Are those operating systems Intel x86 compatible,
then the operating systems fall into three categories:

   * real-mode OSs (eg. MS-DOS and the like)
   * 286-protected mode OSs
   * 386 protected mode OSs
        o using segmented model
        o using flat model

To access PCI BIOS functions, only 386 flat model 386 protected mode OSs (32-bit OSs) do NOT use INT 1Ah.
However the calling program does execute a far call to the BIOS entry point. PCI BIOS supports both 16-bit
and 32-bit calls.At the other hand if there should be extensive testing in the OpenBIOS (POST), then you
are already implementing lots of low-level routines, up to a certain level quite the same as the ints.

> You also need the adapter scan stuff if you intend to use vga or disk
> controllers with on-board bios.
>
> Jim
>

Can't do without ROM scan if you intend to use video above CGA, SCSI or other adapters with onboard ROM.

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Alle Metzlar
Matrix Technologies
P.O. Box 40, NL 1724 ZG Oudkarspel
The Netherlands
Tel: (+31) 226 316889 Fax: (+31) 226 312157
EMail: matrix at xs4all.nl
WWW:   http://www.xs4all.nl/~matrix/  (Matrix Technologies' BIOS Web)
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Author of:
Het BIOS boekje, Uitg. Pim Oets, ISBN 90-5722-013X
Das BIOS Buch, Franzis' Verlag, ISBN 3-7723-4832-7
Die BIOS CD-ROM, Franzis' Verlag, ISBN 3-7723-8612-1
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