I just want to dial in the address of the drive I want to boot from....like we could on the Mainframes....
If you go to the shell prompt, then it takes at least a couple of commands to boot - one to select the drive (the "fs1:" command, for example) and one to start up your operating system loader (often done by invoking elilo with a command line option to select a prepared paragraph out of the elilo.conf file in that directory).
I never did mainframes, so don't know what it's like to "dial in the address of the boot drive." I went straight from IBM 1130's to DEC PDP 8's and 11's, then to sundry microprocessors.