command-option-shift-delete

[Marc Sira]

>> Does this mean that restarting and then holding down
>> Cmd-Opt-Shift-Delete
>> will not result in starting from one of the HFS+ partitions?

> Yes, that's exactly what that key combination does, forces a Mac to look for
> a volume other than the internal boot drive to start up from. This has even
> worked for me when trying to start up from a CD, on those rare occasions
> that holding down the "C" key fails to work.

You can use c-o-s-d (aka "DOCS") or the C key to boot from a different device,
but not a different partition on the same device. For the long version of
that, read on...

Note the distinction between "drive" and "volume". A drive (aka "disk") is a
physical device, while a volume is the filesystem contents of a logical
partition (on some physical device). The four-finger startup combo tells the
boot firmware to skip the internal drive and sequentially try other devices
(really this means it won't look for a driver on the internal disk, which
in some cases in the past could cause the internal partitions to disappear
even once you'd booted an external drive). Clearly this means any partitions
on that device will get skipped. The 'C' key originally meant "go straight to
SCSI ID 3", which was the default place to find a CD drive. Nowadays it likely
means "boot the master device on the secondary ATA controller", which is again
usually a CD.

As for boot order, I *think* it's in ascending SCSI ID order (but it might
be descending, there's a lot of SCSI controller trivia mixed up in my head
from the past twenty years ;). Originally the "internal drive" to skip meant
SCSI device 0 - now it probably means the primary ATA controller.

In troubleshooting disk problems, it's often useful to keep an idea of
layering in your head (as with so many other computing issues, especially
networking). So here we have:

host	controllers (SCSI, ATA, Firewire, USB) that talk to multiple targets
storage	devices and their media (entire hard disks, CDs, removables)
device	drivers that allow the OS to use the physical device
disk	partitions that create virtual smaller devices
various	filesystems (sets of directories and files, all within one partition)
OS	software stored in the filesystem.

The Startup control panel or DOCS key combo lets you choose a device before
any drivers are even loaded (without a driver the device can't actually be
read, which means we don't even know what partitions exist yet). Once loaded
from that device, the device driver will look in the partition table to find
out which partition to boot, or perhaps read yet another key combo for a user
choice - as with the FWB HDT driver (command-option-shift-e-N, where N is a
partition number) or the option key on New World Macs. On newer Macs the
Startup control panel mingles these two concepts, allowing you to choose both
a device and a default partition (your choice is written to the partition
table).

In the boot process, the layers become available in order. That means it
doesn't matter what kind of filesystem a partition contains as long as the
driver can figure out how to boot it - HFS, HFS+, UFS, whatever. Of course,
the OS software stored in the filesystem needs to be able to work with that
type, or the boot won't get very far. But that's a whole other story. ;)

You don't normally have to go very low on that impromptu chart unless you're
erasing or repartitioning a disk. Normal operations, installing software
(even the OS) and the like are all at the highest layer. Disk First Aid or
Norton Utilities mostly work with filesystems (they use "directory damage"
as a synonym for "filesystem corruption"), occasionally fixing partition
table problems. The other stuff is generally hardware hacking. ;)

-- 
Marc Sira		|	toh at victoria.tc.ca
If you can't play with words, what good are they?