[comp.os.vms] Minis as weak links in mainframe systems

MOSS@CS-UMASS.ARPA ("Eliot Moss, GRC A351B, x5-4206 30-Dec-1987 0905") (12/30/87)

Hearing about the Pro-380 front end to the 8550 reminds me of two similar
situations. One is also from DEC -- the 2060 uses a PDP-11 front end to
boot and run diagnostics, and that PDP-11 can also serve a number of
terminals. There can be additional PDP-11s to deal with networks (at MIT,
for example, there was an ARPANet and a ChaosNet connection). It turned out
that if a terminal sent characters to the front end 11 fast enough, the 11
would crash and would think the 20 was down. However, the 20 could be
happily running away and network logins still working fine!

Another, perhaps more interesting story, is that the early Amdahls used Data
General Novas to boot and run diagnostics. In this setup the Nova could
examine and set virutally every flip flop in the Amdahl. Anyway, the point
of the story is that, even with pushing the state of the art in building the
Amdahls, the weak link in the system's reliability was the Nova.
Fortunately, the Amdahl could keep running even if the Nova went down; but
woe if you needed to reboot ...

					Eliot Moss
					Ass't Prof.
					Dept. of Comp. and Info. Sci.
					Univ. of Mass., Amherst