[HECnet] Which DECnet interfaces support remote, unattended, booting?

[Paul_Koning at Dell.com]

On May 17, 2013, at 11:40 AM, Bob Armstrong wrote:

Paul_Koning at Dell.com wrote:
boot support doesn't make any sense in a software
DDCMP implementation ....

I was also wondering if, say, the DLV11 DDCMP driver in DECnet implemented
booting.   As you say, it's not as useful as an independent hardware boot in
the controller, but it would work as long as enough of the OS is still alive
to process DLV11 interrupts.   You've got maybe a 50-50 chance, but even
still - async DECnet was a low end solution so you expect low end results
:-)

The DDCMP driver wouldn't have to actually do the boot - there is a DECnet
boot ROM for asynchronous interfaces already, and you could just assume that
was already set up.   All the DDCMP driver would have to do is start it.

The problem here is that "Boot" has two very different meanings.

Boot.1 == the process of forcing a running or halted CPU to reinitialize itself.

Boot.2 == the process of loading the operating system or main software into a CPU.

For example, boot.1 can be done on a PDP-11 by cycling the power.   Boot.2 on a PDP-11 involves running the "boot loader" ROM code, which loads a small program from somewhere, which in turn loads a larger program... finally resulting in the system being fully operational.

These two are independent in some sense (though the consequence of boot.1 is that boot.2 is initiated).

MOP supports both of these.   Boot.1 is performed by the MOP Boot message.   Boot.2 is performed by the MOP Load messages.

A system may support one or both or neither of these two MOP functions.   For example, it might power up into a boot loader ROM that is configured to run MOP Load.   (Terminal servers are examples of this.)   If so, you might be able to force a restart by sending a MOP Boot message, or that may not be supported.   Conversely, at least in theory, a system might support Boot.1, but what happens next is a boot from disk.

The "boot support" I was discussing -- the one that requires hardware help -- is Boot.1.   By definition it is intended to work when the CPU is not cooperating, so it must be executed in some external hardware, and that hardware has to have the ability to force the CPU to restart.   As John Wilson pointed out, on a PDP11 that means yanking on the INIT line while shielding yourself from the consequences.

Boot roms that support "DECnet boot" are implementations of Boot.2, i.e., MOP Load.

	paul