#MMU disabled?
13 messages in this thread
I just ran AIBB on my 3000/16. It said the MMU was disabled. Is it?
Why? How do I enable it? What is it?
\*donovan*\
Donovan,
MMU stands for Memory Management Unit. It is a chip, on in the case of
your 3000….the chip's functionality has been moved inside the 68030
CPU…that allows the system to have multiple blocks of memory that all appear
to have the same address. That is…it re-addresses memory as far as the rest
of the machine is concerned.
For example, the core operating system on the Amiga is in Read Only
Memory (in most cases) and that ROM resides at a specific location in memory.
You can copy the contents of that ROM into RAM (which also resides at a
specific location or address) where the instructions would execute faster. The
basic problem in doing that is, of course, the ROM and the RAM don't reside at
the same address and the software is designed to look in the address designated
for the ROM when it needs to execute an instruction that is in the ROM.
So what an MMU does is "translate" all addresses that would go into the
ROM, to addresses that are in RAM…thus things run faster. It can also
prevent writes to those memory addresses…so the ROM image (in RAM) is
stable…just as it would be if it were really working with the ROM.
Kind of like when you move, and you give notice to the post office of
your new address. Main sent to your old address gets forwarded to your new
address.
That's one of the main features of an MMU. It's "disabled" because
nothing you are running requires its "services". If you ran a program such as
SetCPU (or it's replacement CPU)…it would be enabled.
Don
Ah, it is a little clearer now. Thanks for the detailed reply.
\*donovan*\
Hello Don,
<<…SetCPU (or it's replacement CPU)…>>
Is there a difference in features or preformance between the two? Just
curious.
-Sean
Sean
CPU is Commodore's program, SetCPU was written by Dave Haynie and isn't
the "official" CBM program (Dave is a CBM employee…or at least he was…don't
know if he got hit in the layoffs).
As to the performance or flexiblity differences between them…no idea.
Don
Dave Haynie is still with CBM.
Steve
Picking nits: Dave works for CISCO (Commodore International Services Co.),
not CBM (Commodore Business Machines, the US sales arm of Commodore).
*Very* few people now work for CBM.
Dale Larson
– An Amiga Software Engineer with some time on his hands.
Thanks for the info, Don.
-Sean
Sean,
if you can get CPU, get it. The documentation is a bit clearer in
places and it does more of the grunt work. Set it up once and forget it.
SetCPU is a very nice program, but it can get downright technical in
places.
Brian
Brian,
I have CPU in my c directory. It's weird how things just pop up and I
have no idea where they came from. <g> Actually, I think it came on the
2.1 ADos disks. I don't have any docs on it that I'm aware of
though…hold on a sec, yes, I do have documentation, it came with the
disks. Thanks for the help.
-Sean
Aside from moving the system ROM into RAM, the MMU on an Amiga is useful
with the AMAX Mac emulator. I think it can also be used by the Enforcer
software, useful really only if you're a programmer.
— Ron Legro, Milwaukee
via AUTOPILOT (the only way to fly)
On my amiga 3000 with AMAX2+ although the setup screen says that MMU is
enabled all of the diagnostics with AMAX say it is NOT. Do you know
something I do not?