Ultrix device names

I have a MicroVAX II that is able to boot into Ultrix 4.3 single user mode. If I type "mount" it says that /dev/ra0a is mounted on /. I assume that is my ESDI hard drive since the drives plugged into my SDC-RQD11-EC ESDI controller are the only ones plugged into my system. So, my question is, what would the second drive on that controller be called? Would it be /dev/ra0b or /dev/ra1a? In other words, what do the "0" and "a" parts of the device name mean? Is "0" the controller and "a" the device on that controller? I also have a TD Systems Viking SCSI controller in the system with a CD-ROM drive attached. What device name would I use for that? I'd also like to know how to address that drive from the MVII boot prompt so that I can boot directly off of the CD-ROM. Thanks, David