[Simh] Simulating a GT40

[Timothe Litt]

On 04-Sep-18 14:14, Lars Brinkhoff wrote:
> Phil Budne wrote:
>> I seem to recall VAX boot ROMs handled by doing "load -r kaxxx.bin"
>> How are VAX boot ROMs done?  At _some_ point I thought it was done
>> with "load -r".  Is that not available in the PDP-11 simulation?
> I randomly checked one of the VAX models.  It seems memory is modeled
> with at least two arrays: M[] for RAM, and a separate rom[].  Digging
> further, I see there's also NVRAM and some other memory regions.  The
> PDP-11 doesn't have this.
>
The Unibus/QBus can have multiple ROMs.  Besides the various M9301
variants (on the Unibus), the 11/34 ASCII console can be loaded as can
the M9312, M792 & M873 variants, MRV11, etc.  And the corresponding QBus
console & boot ROMs.  There are dozens (literally) of ROM variants for
different combinations of devices & consoles.  Not just from the -11
world - LCG created quite a few custom ROMs for PDP-11 based ANF-10
nodes & communications front ends.

If a ROM device is added to the -11, I suggest that:

a) It be capable of multiple units
b) each unit with a start address (in I/O space) & length
c) the unit accept "attach <FILE>" to provide the code/data
d) the existing gt40 hack that Mark described be migrated to use the ROM
e) preferably, provision be made for the other functions of a ROM
module, mentioned below.

Some ROM modules respond to power failure by forcing the trap to 24/26
to use the ROM's address (e.g. power-on boot).  These usually provide a
pin that allows an external switch to force a bootstrap - this is used
by the console ROMs and also by the KL10(DTE)/DL10 to allow the host to
control an -11.  (It's also used by DMC/DMR11s, but in a slightly
different way).  There's a M9312 tech manual on bitsavers...  There's
also a commented listing of that ROM -
http://www.bitsavers.org/pdf/dec/unibus/K-SP-M9312-0-5_Aug78.pdf 
Bitsavers also has an M9301 tech manual.  And some M9301 ROM dumps
turned up at http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/PDP-11_Stuff.html

Attach will accept switches, so you can provide loaders for straight
binary,  ASCII (e.g. S-record or Intel Hex), or whatever else comes to
mind.  I'd start with straight binary (byte 0 of the file maps to ROM
address +0).

With this architecture, adding a boot (or device) ROM becomes as simple
as distributing the ROM image.  And SimH doesn't have to compile-in
every ROM.  And it would bring us closer to being able to handle PDP-11
host and network boot.


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