[Simh] emulating rom in simh?

[Brett Bump]

On Mon, 12 May 2008, Brad Parker wrote:

>
> Tim Newsham wrote:
> >Is there an easy way to emulate a small rom (or map in small regions
> >of additional ram) in simh?
> >
> >The boot procedures for 1st ed unix have a small ROM at 173700 but
> >the RAM only went up to 57777.  We could just populate a full 64k
> >of RAM and laod the ROM code into the appropriate place, but it would
> >be nicer if we could be more realistic in the simulation.
>
> We could make a "diode board" device, which would approximate the
> original device they used.  As I understand it they had a unibus board
> (probably a matrix of diodes) which they hand wired to produce the boot
> code.  Not an actual rom, but worked like one.

An M9301 (if my "own" memory serves me correctly).  This was a quad height
board with diodes scattered across it (the top 2 unibus slots are power).
We had one of these in our PDP-11/20.  I'm not sure if I remember how the
process went, but as I recall, you loaded 173700 into the PC, and then put
the address of the device you were booting into the address register when
you hit start (in our case, 177406, for our Diablo RK03's).  The Diablo
was a very elegant looking drive (half the height of an RK05), but flimsy
when compared to the RK05 (and not as reliable as the RK05).

I'm not sure how you would emulate leaving the console switches set to a
specific address (this presumes you want to emulate the M9301), but doing
this with an M9312 would certainly be possible.

The M9312's that were in our 11/45's and 11/70 had real chips and console
routine that was very much the same as examining/depositing addresses in
simh itself.  Load this one into memory and it should give you the most
realistic simulation I can think of (without building a real console
panel with switches). ;-)

> One way to reproduce this would be to make a device which appears as
> "rom" on the unibus to which you attach a file which contains the
> contents.  Or we could just hard code the contents as a default and
> allow someone to attach a file to override the defaults.
>
> It would be easy/quick to do, I think.  Several of the the current
> devices include 'boot roms' already.
>
> -brad
> _______________________________________________
> Simh mailing list
> Simh at trailing-edge.com
> http://mailman.trailing-edge.com/mailman/listinfo/simh

Brett