[TUHS] Gnu/Stallman (was Bugs in V6 'dcheck')

Arno Griffioen arno.griffioen at ieee.org
Thu Jun 5 17:31:12 AEST 2014

On Mon, Jun 02, 2014 at 10:24:48AM -0400, John Cowan wrote:
> Ronald Natalie scripsit:
> > Still with all it's flaws, on the 286 and later UNIX actually did run in
> > protected mode, something it took ages for DOS/Windows (one can argue
> > backwards compatibility with the early processors) or Apple (no excuse
> > here, the early Macs were 68000's which had protection) to pick up upon.
> The original Mac 128K was a 68000 processor, and IIRC memory protection
> didn't arrive until the 68020.

As mentioned by others the 68010 could, with additional external hardware, 
support memory management.

The original 68000 (and 8-bit data bus 68008) did already have the full 32
bit instruction and data support of the complete family, but for MMU use it 
lacked one critical feature in the fact that it did not push enough page-fault
information on the stack to re-start an instruction in case of an (externally 
signalled) page-fault.

So even if you interfaced external MMU logic then a basic 68000 was still in 
trouble when a page fault occurred as it could not 'start over' the 
faulted instruction.

The 68010 added the correct stack frames to be able to restart a faulted
instruction and also added the first small performance enhancement in the 
form of a 'loop mode' where the CPU could basically cache a small loop 
and execute this without incurring addtional memory wait cycles.

As a result the '010 was usually used in various *NIX machines of the
era like some SUN2's and various machines (one-offs or low production)
from other brands.

Eg. I still have a machine in my collection which is from a small local 
production run that utilises an '010 with a custom, but quite rudimentary, 
MMU based on some simple logic chips and it used to run SVR2. Very slowly 
as it had a whopping 1 Mbyte of RAM and the MMU could give you a virtual 
memory size of 4 Mbyte. 

I did port MINIX to it and added memory management/protection support to 
the kernel. Ran a lot faster :)

With the release of the '020 Motorola delivered their own full-blown (but 
still external) MMU in the shape of the MC68851

Many non-UNIX '020 based machines of the era did not have the MC68851 on 
board at all (eg. most Apples from the time) as it was relatively expensive 
and could incur extra memory latency/cycles being an external unit.

With the '030 Motorola finally moved the MMU onto the same die as the CPU 
(reducing the latency and cost) and it became more prevalent on more 
platforms (although the cheaper 68EC030 was available without an MMU and 
used in many machines)

Bringing this back to UNIX, I used to do some local supporting work at CBM for 
the Commodore UNIX'es that were Amiga, and of course M68k based. 

The official Amiga 2000 '020 turboboard (or one of the A2500UX'es like I have 
at home :) ) does have both the MC68851 MMU and the MC68881 FPU and these
were used for a, mostly in-house at CBM, SVR3.2 based UNIX version. 

AmigaDOS of course also ran fine on it, but did not use the MMU.

The later, general release, SVR4 UNIX version was desgined to run on the
'030 based A3000UX'es, although it still ran on the '020+MMU cards in their
limited (4Mb) DRAM.

								Bye, Arno.

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