On Sat, 26 Jun 2010 00:14:37 +0100, Tim Streater wrote:

In article ,
Bob Eager wrote:

On Fri, 25 Jun 2010 22:14:00 +0100, Tim Streater wrote:

Could you define what's meant by "multiprogramming support" in
this context?

Two operating modes - user and executive. Or did I get it wrong and
the 4120 had that too? Enter user mode with the EXIT instruction,
and system call back with the EXEN instruction...

Yeah, OK, that's pretty common. I guess I never heard it called that
before. But then I never used any ICL kit.

There's obvioously more, if it is to be useful, but that's a good
start.

We had a Honeywell DDP-516 that had a two-mode operation, but it was
useless as (for example) there was no way of telling the previous state
when an interrupt had occurred, so you couldn't restore state. Quite a
lot of other holes too. I rewired the CPU to fix the variouls issues.

Blimey wot was the point of that then (designing it that way, I mean).

You prolly want to be able to designate memory as no-access, read-only,
read/write, and execute-only, too (as well as mapping it).

I remember going to a presentation on the then-new 68000 in 1979 where
folks were asking about such features (and also what you mentioned), but
the Motorola guy said that they figured it would take too much space on
the chip. Turns out that later, when they looked into it, it didn't add
much extra at all.

It was only a little 16 bit machine, predating the PDP-11. Thousands of
them were node processors on ARPANet.

No idea why they did it like that...but history is littered with half
baked solutions. I would guess cost. The 386 didn't correctly trap
certain instructions, it just made them no-ops...which means hardware
virtualisation wasn't possible. That fed through up to Pentiums quite
recently. Now there's yet another operating mode to select to make it
work as it should have done.

(this rather ignores the fact that partial software virtualisation can
actually be more efficient anyway!)



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