Hello Peter!
On 2017-01-09 03:16, peterh...@cruzio.com wrote:
It is my understanding that if the LBA48 property has been introduced to
O.F. "persistently", then a volume which is larger than 128 GB may indeed
be the boot volume.
Unfortunately this is not true. During the boot process, the first
stages rely on Open Firmware implemented functions to load files from a
(boot) volume. Since the OF only supports LBA28, a read or (more fatal:)
write to a position beyond the first 128 GB will fail and, consequently,
the boot will fail.
For Mac OS X, this first stage is BootX. See
http://osxbook.com/book/bonus/ancient/whatismacosx/arch_boot.html
(scroll down, read chapter "Bootloader"). Before BootX loads and
executes the kernel, it relys heavily on OF. And I do not know at which
stage the kernel, XNU, stopps to rely on OF functions, because loading
the kexts (and the kext cache) could still be OF dependant. Once the
KeyLargo kext is loaded, OF is no longer used for ATA disk I/O.
So, why may a bigger partition work anyway? Well, you may just be lucky
and all files required for boot in the OF dependant stage are below the
28-bit LBA barrier.
Nevertheless, using such a partitioning is a poker game. One day, when
the partition is really filled, the operating system may rebuild the
boot files (as it does already for kernel extensions, kexts -- it keeps
a cache) and it will use the next available free blocks on that volume.
The user has no control over this and the boot will "suddenly" fail --
or act weird -- BECAUSE of LBA28 2^28 bits wrapping.
This will definitely happen if you install an update for Mac OS X, say
from 10.4.8 to 10.4.11, and the partition is already filled past 128 GB
of the total size of the HDD.
Also note that Open Firmware will present every volume with boot
information on it that it can access. If a partitions starts before the
2^28 LBA limit and its boot files (which make OF think it can boot from
this volume) happen to be below block 268,435,455, then OF will show it
as a bootable volume. However, such a volume is hugely affected by LBA
block wrapping!
All volumes that start above the 2^28 block limit, bootable or not, will
not be shown (and will not be accessably in any way)by Open Firmware.
But, it is still safer to limit the boot volume to two partitions, the
first of which is 128 GB (131,072 MB, actual capacity) with the remainder
being used for data storage which is not boot-dependent.
It really is the only *safe* way.
Thereafter, the other volumes may be larger than 128 GB.
Yap. Starting at block 268,435,456 you can make one big partition and be
safe.
I had a 512 GB drive and split it 64 GB for Mac OS 9 and 64 GB for Mac
OS X (totalling the first 128 GB with the LBA28 block limit being the
end of the Mac OS X partition) and the rest was one big 384 GB data
partition (which was accessable once Mac OS X had booted with the OF
hack or the modified KeyLargo.kext).
I didn't file a free solution for Mac OS 9 though.
Cheers,
Mac User #330250
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