[HECnet] PDXVAX - 40MB Memory - VAXstation 4000/VLC?

[Johnny Billquist]

Brian Hechinger wrote:
On Mon, Oct 19, 2009 at 04:52:51PM +0200, Johnny Billquist wrote:
Ah, and it might be fun to point out that MIM:: is actually an emulated PDP-11/74.
Yes, that is the multi-CPU PDP-11 that DEC never sold.
I'm usually running MIM:: with only two CPUs online. There is still some debugging to be done of E11 to make it work a little more stable.
How's that working for you?   I've been meaning to set that up myself.

The machine is usually running just fine. There are a few bugs somewhere in E11 that occasionally can bite you, but usually it keeps up and running several weeks between crashes, and I abuse that machine pretty heavily.

One thing that should be noted is that performance can be hurt pretty bad for some applications that map a shared region read/write. RSX turns off caching for those APRs, to avoid cache coherency problems, and E11 do something in reponse to that which can be very visible performance wise. However, not many programs are plying with that, so it's not a big problem. I usually see it when I use my own symbolic debugger for RSX, which are using various tricks to be efficient... :-)

Also, some privileged programs might need recompiling, since RSX with multiprocessor support change the layout of some kernel structures, where offsets move around. If a privileged program is playing around with those structures, it needs recompiling or possibly just re-taskbuilding.

I've learned to write my programs to actually handle the case dynamically, but most programmers never faced the situation, so chances are that programs that to magic things in the kernel might break.

But if you like to play with something a bit different, then I'd recommend you to try it. There is nothing like bringing up a second CPU in RSX. :-)
In fact, the dynamic configuration tool in RSX would make any OS developer today envious. RSX really looks good here. It's an undisclosed marvel. You can take CPUs on- and offline during operation without even blinking. The original idea was that you could then diagnose, repair, and bring the CPUs back online while the machine kept running.
Same goes for memory, buses, disks, controllers, and what else there is.

Makes you wish you had a real PDP-11/74 around... :-)

	Johnny