wich criteria is used by Darwin to set a disk filesystem as disk0s* or disk1s*

[Zeus]

Hi all,
i'm writing you because today i've discovered a too strange thing ...
do you know wich criteria is used by Darwin to set a disk filesystem as disk0s* or disk1s*??

this is what's happened to me with a PowerMac G4.
The G4 has 2 ide disks, a zip drive and a cd-rom.
the disk are a 20Gb quantum and a 60Gb ibm.
the quantum is set to slave and the ibm to master.
both disk has 4 hfs+ partitions.

I've installed Mac OS X 10.1.1 (upgraded to 10.1.2) on the second partition of the master disk (ibm 60Gb) installation and upgrade gave me NO problems, so i've moved the swap to the first partition of the same disk (ibm). Using the df command darwin told me the BOOT disk filesystem was disk1s(from 9 to 12 obviously!), and the 20Gb (disk0s9...12).
Isn't this just a bit strange? the master disk shouldn't be disk0s*????
finished swap moving procedure i've rebooted the mac and checked ... it was OK.
so i have removed the quantum 20 Gb slave disk and booted into os x... df tell me the disk is mapped as disk0s* ... i think that even if this isn't normal it may be rational... the disk count should start from 0 and not from 1!! so i have updated the swap system to disk0s* form disk1s*.
After replugging the quantum 20Gb slave disk df tell me that this (20 GB quantum) disk is mapped as disk1 and the 60Gb as disk0.
This is no more acceptable!!! why at first this 20Gb disk was mapped as disk0 and now as disk1???
And then if i start the G4 with NO DISKS PLUGGED and then reboot WITH THE DISKS PLUGGED the situation return like the start of the story...
60GB -> disk1s9...12
20GB ->disk0s9...12

i've barred darwin is able to detect the hardware disk link on the ide cable (something like the ide CABLE SELECT mode) because changing disks link-position on the ide cable gives no change.

is there a logic in this behavior?

may you help me pleeeease???

thanks in advance