First, in reference to MOP, thgere are in fact 3 types of
MOP boot request; load the OS, load a named file, or load
Diags for the CPU/interface of the machine.

Mop boting normally involves first sending a CPU/interface
specific secondary, then tertiary loader; thats why the
CPU type and interface type fields are in the request packet.
For VMS in a NI cluster, the tertery loader is tertiary_vmb.exe,
and that loads niscs_laa.exe... Note however, that the VMS
boot process is an incestuous bargain between sysboot, sysinit,
sysgen and VMS, by the time vms starts to run, lots of the os
data structures are set up and inited. This give huge flexibility
to how vms is set up on boot. Yeah, and 69 zillion ways to screw up...

The ROM diags of the uV-II and later machines are pretty good.
If they fail, you are running on a broken machine, then the coding
gets VERY machine specific and, ah... fun... The older/bigger
machines, 78x, 75x, 730/725, 86xx, 8[5789]xx booted diags of
the `console media' or used the console cpu to do the diaging. In
fact, on a 780, 8[5789]xx or 86xx, you are very limited in host
CPU diags, alll the diag registers and stuff are invisible to the
host cpu and can only be accessed from the console cpu.

While on the subject, all vaxen have VMB as a boot loader, but
the ROMS son't have the code for devices that never existed in the
machines lifetime. IE, SCSI drives on the uV-II. And no, I not
saying use vmb to boot bsd...

~Paul