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Old 09 April 2009, 20:04   #1
Predabot
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The Amiga had an MMU-chip?? What was it used for?

So I just read the MMU-thread over in the requests, but before I could ask anything, Toni shut it down, fast and mercilessly.

I didn't actually know what an MMU was before, I have to admit... But now I know, it's a part of the CPU, in modern x86 processors, that handles the communication between the RAM and the CPU - the memory handling.

Now, I also read that in some older computer architectures, the MMU is not actually integrated into the CPU's functions, but is a separate chip , or missing altogether. The Atari ST has a separate chip, but apparently that's not a real MMU, somehow?

So, did some of the Amiga-models have an MMU? Was it necessary for some of the higher-end 68k cpu's? Like, does the 68060 need an MMU?

I'm obviously suspecting that the PPC accelerator-cards have built-in MMU's, since those are a lot more complex architectures than the 68k.

Help me out here lads.... explain it to me.
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Old 09 April 2009, 21:35   #2
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The MMU converts one set of memory addresses to another, usually on a program by program basis.

Say program A wants 2Mb. It gets told it has addresses x to y.

The MMU can map that address range to wherever it wants to in the address space, including to hard disk (hence needing the MMU to run virtual memory properly).

Program B can also be given memory including addresses x to y, because the MMU redirects those addresses somewhere else in he address space.


The MMU in Amigas is (almost) always integrated into the CPU. 68030, 68040 and 68060 CPUs can come with one, or it may be disabled due to a bug (EC models). Some high-end Amigas did come with an MMU fitted - the 040 A4000 was one, there may be others, and all Amigas whose CPU can be upgradedd can be fitted with one.

Amigas never needed an MMU, but it is needed to run virtual memory, memory protection (fewer crashes) and most versions of linux.
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Old 09 April 2009, 22:20   #3
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Originally Posted by alphonsus View Post
or it may be disabled due to a bug (EC models).
What do you mean by a bug? I thought EC stood for Economic, as in the MMU had not been added to save costs, lower the price etc?
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Old 09 April 2009, 23:18   #4
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What do you mean by a bug?
A lot of the lower cost chips EC/LC were destined to be full versions but failed quality control. They then had the connections cut to the "optional parts" and became the economy versions. This helps manufacturers save money.

However due to supply and demand sometimes the EC/LC chips are full versions stamped with the economy part number
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Old 09 April 2009, 23:19   #5
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EC arn't made on purpose. they are broken full models that instead of being binned are being sold at a lower price.
Somehow the design of the MMU made it prone to errors and thus there are many EC non MMU chips out there.
Same goes for the built in FPU for 040/060's
They just stamp a diffrent label on it after test shown that these functions were unreliable
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Old 09 April 2009, 23:33   #6
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All Linux distros for Amiga use an MMU. And so do various debugging tools like Enforcer or when using WHDLoad for debugging. As said above virtual memory programs (like Gigamem) use it as do some kickstart mappers.

No Amiga "needs" an MMU to function. Even the high end 060 chips come in LC varients with no FPU or MMU.
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Old 10 April 2009, 01:19   #7
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Toni doesn't believe in MMU's...
...mind you that's ok as there's almost no need for one in Amiga-land.
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Old 10 April 2009, 01:52   #8
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thought the blizzards use the mmu to maprom to fast ram and then speed up rom operations...in which case...it's very handy to have the mmu.
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Old 10 April 2009, 10:13   #9
Toni Wilen
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thought the blizzards use the mmu to maprom to fast ram and then speed up rom operations...in which case...it's very handy to have the mmu.
No, maprom does not use or need MMU.
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Old 10 April 2009, 10:40   #10
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Ok...thats thrown me off, how does that work then Toni?

Why does sysinfo report mmu in use only when rom is mapped to fast ram? (as far as i've seen).
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Old 10 April 2009, 10:59   #11
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Ok...thats thrown me off, how does that work then Toni?

Why does sysinfo report mmu in use only when rom is mapped to fast ram? (as far as i've seen).
It does not use MMU, even 68EC030 Blizzard 1230's with maprom jumper work. Blizzard memory controller does the mapping magic.

I wouldn't trust sysinfo but I guess setpatch or some other program reprograms MMU differently depending on KS location or something.

Try booting without startup-sequence.
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Old 10 April 2009, 11:12   #12
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Well you learn something new everyday...i always thought that was the mmu's main use...especialy when it came to programs like softkick etc.
I know blizz's use their own memory controller for maprom.
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Old 10 April 2009, 11:22   #13
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Well you learn something new everyday...i always thought that was the mmu's main use...especialy when it came to programs
like softkick etc.

I know blizz's use their own memory controller for maprom.
Uhmm? Earlier you said blizzard maprom needs mmu and now you say it does not?

But yeah, MMU was was usually only used for non-blizzard softkicking (A3000 for example required it) and debugging. Amiga virtual memory was a toy because there is no OS-level support.

About MMU emulation: Question still remains: "Why?". Running Linux/Unix on emulation = Just run native version. Enforcer = there are replacements. Virtual memory = TRY AGAIN. Debugging stuff = perhaps useful, still, nobody has said what is required. Full MMU emulation would be nearly useless (too slow). Some kind of "Mini-MMU" emulation would be useful, especially if only data accesses needs to be mapped/trapped.
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Old 10 April 2009, 11:28   #14
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I meant softkick etc...my mistake...i knew blizz's did it with their own controller as it's done by hardware...(maprom jumper) not software....so that's rather obvious that it doesn't use the mmu.....my mistake.
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Old 10 April 2009, 11:49   #15
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you can softkick with even a plain 68000 A500. (provided you have enough memory.)

http://aminet.net/package/util/boot/skick346
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Old 10 April 2009, 11:54   #16
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you can softkick with even a plain 68000 A500. (provided you have enough memory.)

http://aminet.net/package/util/boot/skick346
Of course but we talked about usage of MMU

(and MMU and blizzard maprom softkick is better, memory is protected from writing)
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Old 10 April 2009, 15:54   #17
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didn't Nic Wilson's rom switcher for 040 need an MMU
and some rendering software did use the FPU

Finally I think there was a virtual memory tool on Aminet using a swapfile that required the MMU. All this is nearly 15 yr old memories, I could be wrong
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Old 10 April 2009, 16:36   #18
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If we can add more physical ram up to addressable limit you would not need a swapfile, thus no need for the MMU!

I would like to run the swapfile OFF on my Windows XP config, but stupid memory leaking apps complain when swapfile is OFF and running out of memory even with 2GB of RAM!

Sometimes the MMU or flat memory model of post 486 x86 programs made things worst in the PC world, and the Mac followed.

The Amiga didn't need it and was wonderful with resources.

Guru Meditations are another thing though and I don't know if having a MMU to trap errors would work other than memory related. But isn't AmigaOS running with segmented memory addressing anyways so it doesn't need the MMU?

Last edited by Valken; 10 April 2009 at 19:22.
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Old 10 April 2009, 16:58   #19
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I would like to run the swapfile OFF on my Windows XP config, but stupid memory leaking apps complain when swapfile is OFF and running out of memory even with 2GB of RAM!
read this:

http://blogs.technet.com/markrussino...7/3155406.aspx

You should always have let's say at least 256MB of virtual memory with the regkey set to load more unused kernel memory into there. 32bit apps can use upto 2GB of RAM, so the warning makes some sense. But the difference over 3GB of mem are often nil with windows 32bit. 64bit makes things a lot more interesting tho'
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Old 10 April 2009, 17:33   #20
Toni Wilen
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read this:

http://blogs.technet.com/markrussino...7/3155406.aspx

You should always have let's say at least 256MB of virtual memory with the regkey set to load more unused kernel memory into there. 32bit apps can use upto 2GB of RAM, so the warning makes some sense. But the difference over 3GB of mem are often nil with windows 32bit. 64bit makes things a lot more interesting tho'
Exactly. Page file ("swap file" is the way Windows 9x and earlier used it for. Modern operating systems use it for other things too) should never be turned off, at least without knowing how it works and why it works that way.

IMHO 95% of web "guides" are wrong, blindly following them only makes Windows run much worse in general situations. Optimizing for rare situations is always the wrong way to do.
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