Boot process description (Was: Re: Netboot not detected by system with PCI SCSI host adapter)

[Gero Kuhlmann <gero@gkminix.han.de>]

Hello,

Mike Hammer <mike-hammer@usa.net> wrote:
> So looks like the system ROM BIOS uses the INT19H first and if it
> finds that there's no bootable device in this chain then it will
> call down to INT18H.

The actual process of booting is quite complicated and differs between
the BIOS versions slightly. Supposing you are using a non-BBS compliant
BIOS (which is the most common case), this is what happens:

- After startup the BIOS installs the two vectors 18h and 19h to point
  somewhere into the BIOS itself.
- When all POST tests have been completed successfully, the BIOS searches
  through the ROM area (that's everything from C8000h to E0000h, thus
  excluding the video BIOS and it's own memory) at 2kB-intervalls for
  a certain signature (55AAh). If it find's one, for example from the
  netboot bootrom or a SCSI BIOS, the BIOS takes the length argument
  following the signature and checksums the whole ROM area indicated by
  this length argument.
- When all checks are completed for a certain ROM, the ROM initialization
  routine is called, which can then, for example, redirect interrupts of
  it's own, including ints 18h and 19h.
- After all ROMs have been initialized this way, the BIOS calls interrupt
  19h. If this interrupt has NOT been redirected by another ROM it still
  points into the BIOS. The BIOS int 19h handler then checks to find a
  bootable partition on any drive specified with the BIOS setup. If it
  cannot find any such partition, it then calls int 18h. On early PC's
  from IBM this int 18h was the entry into the ROM BASIC interpreter. But
  for all clones this interrupt has not been used and just printed an error
  message on the screen and hangs the machine.
- If a ROM redirects int 19h itself, it depends on the ROM if it calls the
  old interrupt in case it's own boot routine failed.

Now, there are some BIOS which don't follow this general flow. For example,
a BIOS can check if int 19h has been redirected when a ROM initialization
routine returns, and simply restore the interrupt back to it's original
state. It can then select lateron depending on setup options whether to call
the ROM int 19h or some other portion of the ROM or not call it at all.
The actual process depends on your BIOS implementation.

> PS. In regards to my past postings on testing of Netboot 0.9.0d version,
>     i could send you two SBC's (Single Board Computer) where one has
>     Realtek 8029AS right on the main board and the other having Realtek
>     8139A embedded.  If you think this would help in your development and
>     work toward future releases, just let me know and i'll FedEx it
>     right to you.  I would need your address though.

I have a couple of 8029 cards myself (as they are the cheapest to get ;-)).
Trying to use the 8139 would be interesting, but I still have a 10Base2 Ethernet,
no twisted-pair installation (I don't know which is actually supported by your
boards). Besides, sending the board over the Atlantic just for testing might get
a little expensive. However, thank you very much for your offer - I would
really like to test the 8139 board (and buy it for my own use from you) if
it doesn't get too expensive. Please send me a private email with the board
specs to gero@gkminix.han.de if your offer is still valid.

> PSS. Previously i have posted the block0 froma typical Windows 95 (MSDOS7)
>      boot floppy.  Is this helpful in adapting mknbi-dos to support
>      generating ramdisk from a file as opposed to from floppy?

Sorry, but I haven't had the time yet to check on your boot sector. And as
I haven't anything to do at all, the power supply of my test system broke
today, so I'm now left with no chance of testing the new netboot release
which I'm just preparing (the other systems I own are not suitable for
testing, some are Alpha systems, or network servers). I will have to wait
until I either fixed the power supply myself or bought a new one.

gero.

-- 
You have absolutely no idea what you're talking about, do you realise that?
  - Peter Da Silva
--
Gero Kuhlmann, Hannover     0511/6497525 (Voice)        gero@gkminix.han.de
===========================================================================
This Mail was sent to netboot mailing list by:
Gero Kuhlmann <gero@gkminix.han.de>
To get help about this list, send a mail with 'help' as the only string in
it's body to majordomo@baghira.han.de. If you have problems with this list,
send a mail to netboot-owner@baghira.han.de.