[Simh] Boot ROMs (was Simulating a GT40)

[Timothe Litt]

On 05-Sep-18 06:16, Mark Pizzolato wrote:
> And it would bring us closer to being able to handle PDP-11 host and
>> network boot.
> A ROM could be used to network boot a PDP11 just as it could be used to
> boot from a disk.  The network (XQ) and all disks are already bootable in 
> the PDP11 simulator with the per device boot mechanisms.
You're describing client-initiated boot - e.g. sim>b XQ.

I mean host (network)-initiated boot (and dump), initiated by receipt of
an ENTER MOP message with the correct password.  As in the DMC/DMR
remote boot - see section 3.5.4.2 and the flow chart on p 3-32 of the
DMR11 UG, appendix E of the DMR technical manual, and the M9301/M9312
technical manuals.

The DEUNA/DEQNA use the same mechanism to support remote and power-up
boot, although they also contain additional ROM. 

This is not currently supported.  The non-network (host only) case is
similar.

For -10 comm front ends, a bit in the -10 interface (DL10 or DTE20)
causes the M93xx (or other) ROM to assert AC LO on the Unibus, allowing
the host to gain control of the -11 for load or dump.

Any emulation of these (and there's been recent discussion of it) would
need an equivalent mechanism.

If a ROM device emulation provided an API for this feature, that cause
would be advanced. 

There are two parts to making it work:

Adding an API in the CPU of the form assert_powerfail( vector) - where
the default is the usual 24/26, but a ROM can specify an alternate
(usually its base address + 24/26).    This is common to all initiators.

Getting the ROM, host interface, or network device to call it (or expose
a suitable API) to gain control of the CPU.  This varies by device.

For the -11, the existing boot snippets could be migrated to set the
switches & use the "real" ROMs, though as you point out, this is not
necessary.  Originally, it seemed simpler to extract (or recreate) small
fragments from the boot ROMs than to find emulate all the ROM variants. 
But SimH & its community has grown, and with current demands, moving to
a more faithful emulation would be appropriate.  There's no rush - it
can evolve.  But if the GT40 (and somewhere on my list, ANF-10 network,
plus the attempts at KA/KI/KL) emulation provide the mechanism, in the
long run it would be a better emulation.

For the KS10, the hardware works differently - and calling
assert_powerfail() would be an error that traps to the simh> prompt.




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