[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Re: request for help to make powerpc custom boot disks



what part of:

Mail-Followup-To: debian-powerpc@lists.debian.org
Mail-Copies-To: nobody
X-No-CC: I subscribe to this list; do not CC me on replies.

don't you understand?

On Thu, Oct 11, 2001 at 11:49:27PM -0400, Albert D. Cahalan wrote:
> Ethan Benson writes:
> 
> > not quite, MacOS drivers are not required on floppies, they are on
> > everything else (hard disks, CDROMs etc).  floppies are the only
> > exception and thats the only reason miboot boot-floppies are possible
> > in debian (thats what boot-hfs.bin is).
> 
> Huh?
> 
> 1. firmware reads CD-ROM (therefore must have a driver)
> 2. firmware demands MacOS code
> 3. firmware executes MacOS code ???
> 4. firmware loads the OS
> 
> Um, OK... as long as step 3 doesn't seriously happen, one could
> fake out the firmware with a dummy MacOS driver. I wouldn't
> expect Apple to have PGP signed these drivers.
> 
> Something like this maybe, in whatever binary format Apple used:

you demonstrate how little you know about how OldWorld Macintoshes
work.

first there are two firmwares in OldWorld PCI macs, the first is
OpenFirmware, an extremely and hideously broken implementation of
OpenFirmware, on most machines its either not capable of reading the
disk at all, or can barly read it.

the next is a 4MB ROM with the core of MacOS burned onto it (on very
old macs this was actually complete enough to boot without any other
media).  

the boot process goes like this:

OpenFirmware simply hands over control to the MacOS hardware ROM, this
does not involve disk access in any way, thats why oldworlds generally
don't read the hard disk very well in OF, and thus why quik is such a
pain in the ass.

the MacOSROM does the following (roughly):

searches for all available disks, CDs, floppies etc.
checks the floppy drive to see if it has a floppy, and if so it sees
if its an HFS floppy with MacOS bootblocks, and a `blessed' directory
containing MacOS system files, if all of these are met it executes
MacOS from this floppy.

it then checks the hard disk, the very first thing it checks on a hard
disk (or CD) is the Driver descriptor map (block 0 of the partition
table) this contains a list of MacOS disk drivers installed on the
disk (in Apple_Driver43 partitions), if no drivers are found the ROM
code rules the disk unbootable and irrelevant, thus ignores it[0].  if
the drivers are found the code in them is loaded to replace the disk
drivers that were burned into the ROM (since they are usually broken
and barly able to read the disk).  once it does that it looks for
Apple_HFS partitions, and checks each on for MacOS bootblocks AND a
blessed directory containing MacOS system files, if all of these are
met the first partition it finds is booted as MacOS. (or a
specifically selected partition is, but it still must pass all MacOS
checks).  

the MacOSROM is NOT firmware, its MacOS, the code in this ROM contains
most of the MacOS syscalls and many library functions, its used during
[macos] runtime extensivly.  as i said before it used to BE the OS in
macs, all the disk based MacOS consisted of were patches and bugfixes
that were applied to the ROM based OS.  (how else did Apple fit a
fully GUI OS on a 400k floppy with room for applications), this
archetecture remains in todays MacOS, even on NewWorlds, the only
difference there is the hardware ROM is no longer in hardware, its an
image file attached to an OpenFirmware bootloader which decompresses
it into RAM and passes control to.

now its certainly possible for a disk driver to be written which is
free software, or even pretends to be a driver but really acts as a
bootloader, but nobody is all that interested in learning and coding
such a beast, besides that if you ever boot MacOS it may overwrite
this driver to `update' it (macos upgrades typically do this), a fake
driver would probably interfere with use of MacOS at all, but if you
wanted a macos free machine that wouldn't matter i suppose.

what miboot consists of is a bit of MacOS code, compiled with
expensive MacOS compilers into what appears to be a MacOS System file,
the ROM is convinced enough that its MacOS that it will accept it and
boot it, but only if the other checks pass (the MacOS bootblocks
present on the disk, and the MacOS drivers are loadable from the disk)
Real MacOS isn't always fooled and renders miboot unbootable by
removing the bootblocks and blessed directory bits since it can
usually tell whats Real MacOS and whats a fake.

BenH will also tell you of all the troubles of MacOS based bootloaders
and the problems it causes.  (miboot is a MacOS based bootloader).

your thinking of quik and OpenFirmware, it does not require any MacOS
drivers, HFS partitions or any of that crap, the problem is OF is so
broken its a pain to get it to read the hard disk, much less a floppy
or a CDROM. (floppy is sometimes more reliable then hard disk).

[0] there are a couple IDE based oldworlds that the MacOS ROM did not
demand have drivers installed on the hard disk, this is because Apple
shipped those without a partition table at all, just a HFS filesystem
put right on the raw full disk.  this is a very few machines though,
by far the majority all REQUIRE valid MacOS disk driver partitions for
the MacOSROM to consider them relevant.

-- 
Ethan Benson
http://www.alaska.net/~erbenson/

Attachment: pgpsLFd4YCpmp.pgp
Description: PGP signature


Reply to: