[Simh] Fwd: emulating rom in simh?

[Carl Lowenstein]

On Mon, May 12, 2008 at 7:16 AM, Brett Bump <bbump at rsts.org> wrote:
 >
 >  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.

 About as Read Only a Memory as you can get.  :-)


 >  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).

 M792, as you can read in the Field Guide to Qbus and Unibus Modules.
 The 11/20 backplane could fit modules only in slots CDEF because the
 fans blocked slots AB.


 >  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.

 There were enough spare words in the diode ROM to preset one device.
 Just cut the diode leads to program the bits to one from the default
 zero state.



    carl
 --
  carl lowenstein marine physical lab u.c. san diego
  clowenstein at ucsd.edu



-- 
 carl lowenstein marine physical lab u.c. san diego
 clowenstein at ucsd.edu