[net.micro.68k] 68020 MMU Questions

wall@fortune.UUCP (Jim Wall) (11/20/84)

    Is anyone planning on using the Motorola PMMU chip? And if
so, what about their emulation of this chip, the MMB board?

Come on Sun and Dual, or are you still using propriatary MMUs?

Anyone looked extensively at the MAC from Signetics? To the best 
of my knowlege, it isn't any more available than the PMMU. Nor is
their gate array synthesis any more available than Moto's MMC.

   Am I the only one experiencing frustration with the MMU question
for 68020 support? I suppose I should just impliment our own MMU 
again, and ignore the VLSI....

Let's have it, respond to the net or to me personnally. 

					Thanks much.

					-Jim Wall

					...!amd!fortune!wall

fair@dual.UUCP (Erik E. Fair) (11/21/84)

DUAL Systems has never used a proprietary MMU scheme for any of the
68000 family of CPUs in any of our boards (unless you consider the
68451 from Motorola to be proprietary).

The very first DUAL UNIX system had no MMU because we waited for
Motorola to finish the one they were working on. In retrospect, that
was a mistake.  In the meantime, however, we just split memory into two
sections, one starting at location 0, and one starting at location
0x800000. The UNIX Kernel ran in the 0 section (or `kernel memory' as
we called it), and user programs ran out in the memory at 0x800000.
When a program in user mode attempted a reference to an address less
than 0x800000 (i.e. without address line #23 on), the system took a
protection trap, and we would blow the user away with SIGSEGV.

Needless to say, without an MMU, context switching was a major chore
(lotsa copying of user text, data, and Udots) and was *very* slow.
However, UNIX worked.

When the 68451 arrived, it was designed into the CPU board, and our
kernel was modified to take advantage of it. I don't think there are
any of the non-MMU DUAL UNIX systems left out there. (I certainly hope
not!).

	[begin disclaimer]

	This is soley my opinion, and does not necessarily reflect the
	opinion or policy of DUAL Systems Corporation.

	[end disclaimer]

	Erik E. Fair	ucbvax!fair	fair@ucb-arpa.ARPA

	dual!fair@BERKELEY.ARPA
	{ihnp4,ucbvax,hplabs,decwrl,cbosgd,sun,nsc,apple,pyramid}!dual!fair
	Dual Systems Corporation, Berkeley, California

schoff@cadtroy.UUCP (Martin Lee Schoffstall) (11/26/84)

> 
>     Is anyone planning on using the Motorola PMMU chip? And if
> so, what about their emulation of this chip, the MMB board?

CADMUS is not going to use the Motorola PMMU chip.

> Anyone looked extensively at the MAC from Signetics? To the best 
> of my knowlege, it isn't any more available than the PMMU. Nor is
> their gate array synthesis any more available than Moto's MMC.
> 
>    Am I the only one experiencing frustration with the MMU question
> for 68020 support? I suppose I should just impliment our own MMU 
> again, and ignore the VLSI....
> 

I believe that for those interested in building virtual UNIX machines
the tradeoffs are availability, cost, and performance/functionality.
I believe this is what you find frustrating, finding the optimal point on
the graph of the function is highly company dependant.

marty

{wivax,seismo,bbncca}!cadmus!schoff
cadmus!schoff@seismo.ARPA