This valuable information was left as a posted comment on our Contact page by user Madcat on January 13, 2021:
Hi, You may have figured this out already but I heard your comment about Disk identifiers in BTOS/CTOS so thought I'd add my two-penny worth of memories.
The OS supports multiple physical disk drives, these are addressed linearly along the X-bus, so the first hard disk encountered is D0, the second is D1... likewise the first floppy is F0, the second F1... When a Disk is intialised it is also assigned a name, and if I remember correctly the boot disk is always called 'Sys' (I think earlier iterations called it 'Win' and I can't remember if this was then retained as an alias but hey-ho! :-)) so, the upshot of this is that you can address a disk either by its position on the bus or the name you give it when it's IVolumed so if your boot disk is the first disk on the bus then you can refer to it in any command as either [d0] or [Sys] (and possibly [Win]).
If you have a cluster workstation with a local file system then if you wished to refer to disks at the master you prefix the drive/name with an exclamation mark so [!D0] or [!Sys] would refer to the first disk at the master or the boot disk at the master respectively (they needn't be the same but were usually). The OS is also transparent to this so you can use this nomenclature even at the Master workstation thus referring to [!D0]myrunfile.run in a command run there just points to the Master workstation d0 disk at any location in the cluster.
Hope that rather sketchy explanation helps - might help you with the boot emulation issue you were referring to. The file system has some other nice addressing quirks too but I won't bore you with them.
