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 Post subject: Re: Amiga!
PostPosted: Sun Mar 22, 2020 12:41 am 
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I believe that Solar was referring to the 1200, which did have a 68020EC, unlike the earlier 500. But the EC version of the CPU didn't have an on-board MMU.


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 Post subject: Re: Amiga!
PostPosted: Sun Mar 22, 2020 5:54 am 
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Thanks guys. So the actual CPU used didn't have an MMU, and when MMUs were available the usual problems of compatibility with an existing codebase got in the way. It makes sense now.

Changing bus size is a big deal, isn't it? I want to go all 64-bit with my system, but still have some 32-bit hardware I'd like to run.

I forgot to reply to this yesterday:

Solar wrote:
The greatest feature -- which kept Amiga in the market long after Commodore croaked -- was the Expansion Port, which routed all the necessary signals to have newer CPUs on expansion boards.

Yeah, replacing the CPU is awesome! Atari 800 had a good expansion bus, but not that good. :) You could override memory access but not the CPU. Actually I might be wrong about that; it depends if DMA lines were brought out to the port... but no, there weren't the 2 DMA lines needed to stop the 6502 and still receive DMA signal from the ANTIC. It's probably possible on the 130XE by using RAM not currently bank-switched for video. OTOH I don't think it's possible to RAM-expand the 130XE without soldering internally. It is possible to plug RAM expansions into the earlier models. Hmm... with external memory, an external CPU could be made to work, but still requires a lot of thought on how to synchronize it with the DMA & everything else. I suspect I'd find it easier to design my own computer with video output and all.

Thinking about all this made me realise what a lot goes into an expansion port like the Amiga's!

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 Post subject: Re: Amiga!
PostPosted: Sun Mar 22, 2020 10:54 am 
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iansjack wrote:
I believe that Solar was referring to the 1200, which did have a 68020EC, unlike the earlier 500. But the EC version of the CPU didn't have an on-board MMU.


Sorry, that was what I was trying to say in that sentence, but somehow I dropped that part. The second sentence of that paragraph related to why AmigaOS didn't have support for the MMU initially, and probably should have been in the next paragraph.

Now I feel like an utter idiot for not being clear about that. Again, sorry.

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 Post subject: Re: Amiga!
PostPosted: Sun Mar 22, 2020 1:00 pm 
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Schol-R-LEA wrote:
Now I feel like an utter idiot for not being clear about that. Again, sorry.

No worries. Human memory is a bit like dynamic RAM, facts which don't get refreshed periodically get corrupted. The mechanism is different: the actual synapses between neurons are like muscles, they get stronger with use and weaker with disuse. It's been a long time since the 68020 was current, so weak synapses & corrupted memories are to be expected. :)

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 Post subject: Re: Amiga!
PostPosted: Thu Oct 15, 2020 6:27 am 
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I still have a bunch of Amiga machines, and even wrote an Amiga emulator a few years back. https://github.com/h5n1xp/Omega :D

I always enjoy chatting about this enjoyable part of my past!

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