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gt
08-18-2001, 02:23 AM
In ed374's thread one of the troubleshoots was to start up while holding
the Command, Option, Shift and Delete keys (COSD). Las Vegas (in a reply) pointed out that this keyboard shortcut told the machine to ignore the selected start up drive.

Teaching time please-

What happens next?

Does the Mac poll the other devices looking for a system folder?

If so, in what order? Does the device ID number come into play?

I'm starting from a Mirrored RAID (Miles2, Cheetrahs, SCSI bus 2). If I do not have a system disk, or utility with a system folder in the CD will my
machine look on my "emergency" start up drive (SCSI bus 0, ID #0) for the system folder that resides there?

If I DO have a system or utility CD in the CD-ROM drive (SCSI bus 0, ID3), will the COSD command act like the "C" key when IT is held down on startup and go directly to that drive?

Then, just to make sure that I make this as difficult as possible (sorry),
What if there are two partitions with system folders on each of two drives or volumes?

If you could point me to a place that might have the answer I'd appreciate it.

I just gotta know.

dragon_x
08-18-2001, 11:05 AM
Does the Mac poll the other devices looking for a system folder?
Yes!

If so, in what order? Does the device ID number come into play?
Good question - This is my guess - but it probably depends on the system. You only have SCSI (stock) in your 8600 - so my guess is that it would look on the internal SCSI bus and start with SCSI ID #0. Normally by default your boot drive (OEM from Apple) would be SCSI #0 - so it may check this anyway or it may skip it or it may skip whatever was it was originally set to boot with. It may also depend on the firmware/ROM chips in the system and any PCI cards... the new Macs can boot off of - IDE/ATA, FIREWIRE, USB (?), and SCSI - but not SCSI RAIDs....so there you go.


I'm starting from a Mirrored RAID (Miles2, Cheetrahs, SCSI bus 2). If I do not have a system disk, or utility with a system folder in the CD will my machine look on my "emergency" start up drive (SCSI bus 0, ID #0) for the system folder that resides there?
That would be my guess. Even if you had a system folder on a CD - that would not change the computers search order because you havent hit the C- key.

If I DO have a system or utility CD in the CD-ROM drive (SCSI bus 0, ID3), will the COSD command act like the "C" key when IT is held down on startup and go directly to that drive?
I do not think so. I really never use the CDOS command because:
1) I can boot off other drives - by setting it in the startup control panel.
2) I can boot without extensions.
3) I can boot upto the extensions manager and kill any offending extension
.4) I can put a bootable CD in the CD-ROM/DVD drive.
5) Physically disconnect a drive you DO NOT want to boot off of.

It is rare that none of the above will work.

I think I only had to use CDOS one time - in over ten years of computing on Macs. I know with the clones and esp - FWB CD toolkit that CDs became very problematic - but I often insist on getting Apple bootable drives and Intech Speed Tools or the Apple hacked drivers work pretty good.

Then, just to make sure that I make this as difficult as possible (sorry),
What if there are two partitions with system folders on each of two drives or volumes?
No need to be sorry - these are good question... but most people probably wont need them http://macgurus.com/infopop/emoticons/icon_smile.gif (knock on wood! http://www.macgurus.com/ubb/eek.gif ) I would guess that the Mac will check the ENTIRE drive - that is ALL parititions (or all mountable partitions) to see if there is a system folder. There are exceptions - though - the bG3 (gossamer) will not boot off a partition that is outside the first 8GB. The systems before this (604e/603ev) and after (Yosemite) do not have this issue. I would imagine Apple set the scan up so that a boot drive is found quickly - so the mostly likely location will be searched first (SCSI bus #0, SCSI ID #0) - for vintage Macs and ATA/IDE bus #0 - Master drive for GossamerG3 and newer Macs.

I would imagine this is similar to clearing the PRAM/NVRAM - since clearing this will force the Mac to scan the buses to for the first bootable volume. Of course with the CDOS command you can force it to go to other drives.

I'm sure someone else will know more... but this should keep you busy for a few seconds http://macgurus.com/infopop/emoticons/icon_smile.gif

------------------
Life in the fast lane leads to:
The Resteraunt At The End Of The Universe

gt
08-18-2001, 02:00 PM
Thanx for the reply d-x,

I've never used the COSD combo. (Repeat Dragon_x"s wood knock).

Is the ROM version on the mac, ($77D.34F2 in my 9600),
the governing factor in determining the order in which the drives are "polled" for a system folder when using COSD, or is this something that the OS stores in PRAM or NVRAM for use?

I figure it's gotta be kept somewhere so it can be accessed before the system folder is found. Am I close?

Thanx - Greg

lasvegas
08-19-2001, 03:05 AM
The default boot drive on a Macintosh is SCSI 0 - ID 0. This remains unless a different drive is selected in the Startup Disk control panel. If a System Folder isn't found in the Startup drive (Or the drive is blocked via Cmd-Opt-Shift-Delete), the system will pole each drive starting with ID 0 and then down from ID MAX (6 on the internal bus) to ID 1. If no System Folder is found, then the same pattern is tested in each additional SCSI Bus from the lowest to the highest. Then the IDE busses are polled in the same fashion (0-Master, 0-Slave, 1-Master, 1-Slave, etc).

Now the Command-Option-Shift-Delete option instructs the Mac to ignore the currently selected startup drive (All of its partitions) until another System Folder is found or the keys are released. This can be handy if you currently have a bad driver on your boot drive and have another boot drive. If you keep the keys pressed throughout the boot process the current boot drive won't even mount! You can then proceed to install a new driver on the unmounted drive.

Note that if PRAM has been Zapped or the CUDA switch pressed, there is no Startup Drive selected. Therefore the polling process is executed everytime. This is most apparent on New World machines because you'll see a flashing question mark folder (No Startup System Found) until a bootable drive is polled.

The 'C' key works a bit differently. It instructs the computer to recognize the default CD drive as the boot device (SCSI 0 - ID 3 or IDE 1 - Master). It will then check the CD for a System Folder. If none is found, the drives are then polled as above.

[This message has been edited by lasvegas (edited 19 August 2001).]

gt
08-19-2001, 03:53 AM
Thanx, lasvegas! Very clear and concise.

You are a welcome addition to the expertise already extant in these forums - especially for a guy like me who's playing catch-up.

Where do these "Polling instructions" originate? In ROM? Or ???

Greg

lasvegas
08-19-2001, 04:15 AM
The polling code is part of the bootstrap code in ROM. The default boot device, if set is located in PRAM. There are ways of making minor patches in the bootstrap code in NvRAM using Open Firmware, but isn't retained between boots in OF 1.05 (The version in your Mac and mine).

This is the technique XLR8 uses to enable the backside cache before booting. To get around the clearing of the code between boots, XLR8's software rewrites the patch everytime its extension loads.

gt
08-19-2001, 12:34 PM
Gracias http://macgurus.com/infopop/emoticons/icon_smile.gif

Greg