Re: partition table type

[Bob McGowan]

Mike McCarty wrote:

Bob McGowan wrote:

Johannes Wiedersich wrote:

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jeffry s wrote:

i am also wondering what all those partition type mean. can anyone point

any
documentation about
the difference of amiga, bsd, dvh, gpt, mac, msdos, pc98, s390,
sun, loop?

It's just a different format of how the start of your hard disk is
organized. I don't know about particular differences.

If you can't find it on google, it is probably so obscure that you won't
ever need it. Look at the article in wikipedia on partition types.

You only need a different partition type than the default, if you have
an other OS on the same hard disk that doesn't understand the default

Is this the whole story? I would have thought that the base hardware would also have something to do with it.

What do you mean by "base hardware"?

For example, the BSD OS versions designed to work on Intel architecture would need to have disk partitioning that the BIOS could work with, just as for msdos. And modern PC BIOS support for gpt is becoming common, too.

The BIOS has nothing to do with disc partitions. The BIOS works either
at an absolute physical level using geometric address: cylinder, head,
sector, or at a logical physical level (LBA) using logical sector
number. It has no other conception of the disc.

But, a disk that is fully mac oriented, for example, could not just plug in to a PC system and boot. The system would recognize the disk as

Of course not, if by "fully MAC oriented" you mean having a PPC on it.

available, but would not be able to do anything with it, as the partitioning is totally foreign (though, with the Intel Macs, this may no longer be completely true).

The partitioning doesn't enter into it.

But when you say amiga or s390, I'd think you were explicitly loading an amiga or s390 version of an OS (which could be Linux) on Amiga or S390 hardware. The basic hardware and it's equivalent of a BIOS would then dictate the partition type to use.

I don't know how boot load works with those machines. Here's how it
works on PC architecture machine

The processor comes out of reset in real mode, and sets the IP to
point to the reset location (I forget the address, but perhaps
FFFF:8000).

The BIOS is now in control, running from ROM. It does basic tests of
functionality and preliminary initialization of hardware. It may
switch to priviledged mode type operation. It enumerates the PCI bus,
and if it finds bridges, initializes them and enumerates sub PCI busses.
If it finds peripherals it recognizes, like USB I/F or hub, it
initializes them. Then it looks for things like video cards, keyboards,
etc. on the back side. If all looks good, it "beep"s and then starts
to look for something else to boot. If it switched away from real mode,
it switches back.

The BIOS now looks for bootable devices, using some order for device
type and device ID. Typically, it looks for CDROMs, floppies, and
hard discs, in that order. Possibly for USB sticks.

I am familiar with how floppy and hard disc boot works, and how some
CDROM boot works.

For a floppy or hard disc, and for some CDROMs, the first absolute
sector is loaded into memory at location 07C0:0000. If the last
two bytes are the special marker AA55, then this indicates bootable
device. If not, then the device is skipped. If the marker is
present, then the processor jumps to the first byte of the loaded
sector, which then assumes control of boot. For a floppy, and for
certain CDROMs which just have an image of a bootable floppy on them,
this is the BR. For a hard disc, this is an MBR, and the code in the
MBR must find an appropriate partition.

For a hard disc, the code in the MBR scans the portion of the MBR
which contains the PT looking for partitions (also called volumes,
not to be confused with LVM) which have the "active" or "bootable"
bit set in them. If none is found, or if more than one are found,
then an error has occurred. Else, the code in the MBR has found
what purports to be a bootable partition. It loads the first
sector from that partition into memory at location 07C0:0000, and
looks at the last two bytes. If they are AA55, then it presumes
it has found the beginning of a volume, which contains exactly
the same information as the first sector of a floppy. IOW, it
has found a BR, and we are in the same situation as with a floppy
and certain CDROMs.

At this point, either the BIOS or the MBR code, whichever has control,
has loaded a BR, and jumps to the first instruction. The BR has in
it a thing called the BPB, which describes the geometry of the
floppy disc, floppy disc image on CDROM, or of the partition of the
hard disc. The code in there looks for a file system, tries to find
the appropriate boot images, and loads, initializes, and starts
that boot image. What happens during this is OS dependent. For
MSDOS, for example, MSDOS.SYS (actual OS) and IO.SYS (BIOS extensions)
get loaded and COMMAND.COM gets run.

msdos. You should check the documentation of all your other OSes to find
a partition type that is supported by all of them.

It's good to know that debian also supports obscure stuff :-)

Mike

--
Bob McGowan

Which is why I asked the question. I'm not a hardware engineer or firmware developer, so having others, more knowledgeable than myself in those areas provide helpful info, is good. To summarize: Power on reset; jump to and run BIOS; find bootable device by reading first absolute sector of a disk device; based on the value AA55 as the final two bytes, jump to and run the data just read into memory. Up to this point, nothing is specific to any disk partitioning scheme or OS. If that last bit of code reads an MBR that understands an s390 partitioning scheme, and the code stored in the partition is appropriate for the CPU of the system, it should boot and run?

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