command-option-shift-delete

[ELN/rlf9]

Duo/2400 List wrote...

>From: Marc Sira <toh at victoria.tc.ca>
>Message-Id: <200301142150.h0ELowX8011094 at vtn1.victoria.tc.ca>
>Subject: [Duo2400] Re: command-option-shift-delete
>Date: Tue, 14 Jan 2003 13:50:58 -0800 (PST)
>
>>> 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. ;)
>
>-- 
Aha! I see! Thanks!

Bob F