Subject: Re: Retro computing to an extreme?

From: [email-masked] (Don Stokes)
Date: 22 Dec 93 08:30:51 GMT

[email-masked] (Charles Lasner) writes:
> Your belief doesn't match the situation.  After loading the WCS, the
> machine is no longer an -11, and doesn't even run RT-11, much less RSTS. 
> Assuming a composite was developed, what's the overhead in allowing some
> -11 cycles and some -8 cycles?  The whole point of the hack was to run a
> faster -8, not a slower one.

<sigh>  I know you don't like to admit the pdp11 is useful, Charles, but...

The WCS allows you to extend the instruction set, and there is a lot of
free space in the -11's instruction set that can be used for new
instructions.  In fact, if you don't mind not being able to use an FPP
you can use the whole 17xxxx range, which happens to be a full 12 bits
wide.

(Note that the pdp11 has a standard set of FP instructions that are present
on most processors, including the 11/60.  The FPP was available as an option
for the larger -11s, and is by no means a requirement for most software to
run, certainly not operating systems.  Just because you don't have an FPP
doesn't mean you can't do floating point -- it just goes a bit slower.)

So, you can then emulate to your heart's content, simply by stuffing the
top four bits of the -8's address space with 17 and having the -8 microcode
only operate on the bottom 12.  

That leaves the rest of the instruction set untouched so that the OS can
run.  This just leaves the question of I/O -- and mapping -8 disk
instructions into RSTS/E file system or block I/O calls onto an -8 
formatted disk or container file isn't really terribly difficult. 
Anything too hard for the -8 microcode can just be punted to a pdp11 trap
to be dealt with.

Running under RSTS/E would give the illusion of multiple virtual pdp8s.
The worst case for development would be simply to develop using pdp8 tools.

> 2)	All of the files would have to be tediously moved across from an
> -8 on RX01/02 and then reconstructed on the mythical disk on the

Filesystem conversion is trivial.  Look at all the foreign filesystem
utilities floating around for practically every system on the planet.
You'd do most of your work on the faster -11 and only move back onto
the -8 for final testing.  

Anyway, much of this is idle speculation -- whether it was done or not
(and I still believe it was, at least for later development of the micro
versions) it is certainly possible and even fairly likely given that the
11/60 microcode hack was in fact done.

--
Don Stokes, Network Manager, Victoria University of Wellington, New Zealand.
[email-masked](home) [email-masked](work) +64 4 495-5052 Fax+64 4 471-5386

---

From: [email-masked] (Charles Lasner)
Date: 22 Dec 1993 21:59:49 GMT

>you can use the whole 17xxxx range, which happens to be a full 12 bits

You are changing the problem in midstream.  The original notion was
whether someone was loony enough to actually use an 11/60 to develop WPS. 
You are guessing why you feel it's worth doing.  I am providing reasons
why logistically no one would've bothered to beyond demonstrating the hack
of an unloaded 11/60 could be demoed running -8 code at 2-3 times the
speed of an -8/e.  The overall development requirements preclude that it
ever happened.  Any existing or proffered utilities you suggest wouldn't
be as useful as the dozens of existent -8's the developers had as a much
more viable alternative.  The overhead of even considering such a stunt
more than cancels any alleged benefit.

>only operate on the bottom 12.  

Perhaps true, but the demo was for a faster -8, not one convenient for the
host WCS -11.

>formatted disk or container file isn't really terribly difficult. 

You underestimate the overhead of perfect emulation of the PDP-8
peripherals required by WPS.  OS/8 is another story, since it runs without
interrupts and only needs block service, etc.  Again, unless it's both a
perfect timing-wise emulation *and* also fast, it's worthless for this
purpose.

>Anything too hard for the -8 microcode can just be punted to a pdp11 trap

Which isn't useful to the stated task.
>The worst case for development would be simply to develop using pdp8 tools.

Multiple developers of any application divide down the alleged performance
boost.  Better overall to just go use a stand-alone -8.  It's guaranteed
to be 1/2-1/3 of the best-case expectation of the RSTS system according to
your own idealistic notions.

>the -8 for final testing.  

Totally wrong.  There is no justification in using the -11 filesystem at
all.  It totally lacks the development tools needed, except for an editor,
which OS/8 itself has.  The entire -8 development effort *must* be
performed under emulation.  People aren't going to tolerate large amounts
of wait times moving floppies back and forth, especially when a single RK
pack solves all problems!

>11/60 microcode hack was in fact done.

Sorry to burst that bubble as well.  All the DECmate versions are straight
OS/278.  The venue moved to RX50, and everyone had one in their own
cubicle.  Way too much trouble to even think about it.  Besides, how late
do you think DEC even had in-house WCS-capable 11/60's running?

cjl