🚀 go-pugleaf

In article <738@dlvax1.datlog.co.uk>, rch@datlog.co.uk (Richard Hughes) writes:
> Can anyone tell me :
> a) Why would one want to change the boot device in this way ?

 i) {why change boot device?}  There are lots of reasons, but most typically,
    in my experience, to have more than one operating system installed, e.g.,
    both AIX and 4.3/RT, as I have on one of my machines.
ii) {why this way?}  (Because the engineers decided to do it this way??)

> b) How and why has it been changed ?

The ROM default boot order is floppy 0, floppy 1, disk 0, disk 1, disk 2.  The
boot code will look at the NVRAM ("Non-Volatile") and if there is a valid CRC,
it will use the boot order specified in bytes 27-31.  If you change the boot
order and then od -x /dev/nvram, you should see f0, f1, d0, d1, d2, respectively
for the selected boot devices.  If the boot code finds such entries, it won't
try the unlisted devices.

In AIX, the special key sequences Ctrl-Alt-A, Ctrl-Alt-B, ... will set the
initial value in the NVRAM (byte 27) to f0, f1, ..., respectively.  Thus
Ctrl-Alt-C would say that it is only possible to boot from disk 0, for
example.  After the VRM boots, it will still check for a bootable virtual
machine on floppy before it tries for a bootable virtual machine minidisk.
This is why install/maintenance would boot but not the diagnostics, if the
NVRAM was set to only boot from disk.  I've only tried these sequences in
normal operation modes, they may not work at some other times.

In 4.3/RT, the sautil diskette has a utility for changing the boot order in
NVRAM.

Sometimes I will use one of these (key sequence or sautil) to choose whether
to boot AIX vs. 4.3, but usually I just leave things defaulted to boot AIX 
and boot 4.3 from sautil when I want to run 4.3.

> c) Having changed it, is there a better way to reset it ?

I normally have found either the key sequence or sautil approaches sufficiently
usable for the few times that I've really used this feature.  Pulling the
battery also works.  In AIX, I imagine that if one tried writing to /dev/nvram
(with root authority), this could be made to work.  There may be some equivalent
special file in 4.3/RT, but I haven't noticed it.
-- 
Charlie Sauer   IBM AES/ESD, D75/802     uucp: ut-sally!ut-emx!ibmaus!sauer
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