saw this on the Darwin list, thought might be useful.

Jake

---------- Forwarded message ----------
Date: Thu, 30 Nov 2000 09:49:00 -0800
From: Josh de Cesare <decesare@apple.com>
To: Darwin-Development@lists.apple.com
Subject: Re: OldWorld boot

James,
	Old world booting begins with the boot-device being set to 
<device_path>@<target_id>:<partition_number>.  One of the patches to 
mac-parts cause OF to ignore the partition number and use zero. 
When OF tries to boot off of a partition number of zero it will 
search the partition map entries for a partition for one that include 
a "PowerPC" driver.  The entry will include the location and size on 
disk, the location in memory and the start address.  OF loads this 
"driver" an executes it.  For Mac OS X or Darwin this is an unpacked 
version of /usr/standalone/ppc/bootx.xcoff.  Once OF calls BootX 
there is little difference between New World and old world machine.
									Josh