More PDP 11 hacking (help!)

From: Pete Turnbull <>
Date: Tue Feb 8 15:12:29 2005

On Feb 8 2005, 11:06, Vintage Computer Festival wrote:
> On Tue, 8 Feb 2005, Pete Turnbull wrote:
> > > But now are new woes: when I type in "DY" to boot off the DY
> > > the system pauses for a split second and then dumps into ODT with
> > > 165426.
> > > Looking at that address shows that there's no code there.
> >
> > No code, as in ODT gives an error (prints a "?" and a carriage
> > return) or it's zeros?
> No code as in the memory contents are the default at power up
> (alternating between 0 and 177777, I think(.

Interesting -- this is an 11/23, yes? It presumably has 64KB of
memory, or more. However, what a program can see is at most 56KB of
addresses, and the remaining 8K is the I/O page. When you look at
165426 with ODT, you see the physical memory, but when the processor
does it, any address above 157777 is mapped to the I/O page. To see
what the processor would see while it's executing the bootstrap, you
would need to look at 17765426. Unless it's one of the early KDF11-As,
with only 18-bit ODT, in which case you should look at 765426 (17765426
would actually give the same result, though, because of the way ODT

> > Depends on what's at location zero. You could try entering a copy
> > of the normal RX02 bootstrap code at location 1000 and stepping
> > through it to see what happens and where it goes wrong.

> Yes, I was aware of that but I'm not sure how useful it would be to
> at this point since I'm not versed (yet) in PDP 11 machine lagnuage
> and wouldn't know what I should be looking for.

If it helps, I can give you an annotated copy of the RXV21 bootstrap.
 I've posted it here before, but I can do it again. I've got other
noddy toggle-in programs, for example one that prints out all the valid
addresses in the I/O page, and I have a listing of the standard
addresses and what uses them.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Tue Feb 08 2005 - 15:12:29 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:37:36 BST