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Atari coding BBS
| Re: PMMU question |
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Posted by: dml
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Mar,14.2014-13:03
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Hi Laurent,
Mikro is right - you won't see much benefit because the PMMU has a cache of its own (ATC or address translation cache), and the tree itself is not big, dividing memory into quite large regions. IIRC there are only 2 or 3 tables of 16 entries each to cover the whole 16MB address range.
I don't remember the size of the ATC (consult the 030 manual for that) but I guess the only way you could 'thrash' this cache using such a small tree is to randomly read lots of different regions in turn, separated by at least 256k or so, on each memory cycle.
In practise though, you tend to read/write a lot of stuff to approximately the same area, and don't switch to other areas frequently. Even a complex drawing operation probably uses no more than 3 or 4 memory sources in separate places and the ATC should handle switching between those easily.
However I did measure a (very, very) small gain by turning the tree off (in fact, turning the transparent translation mode on, which looks past the tree) and it wasn't clear if this was due to the ATC missing sometimes, or just having the tree 'on'.
The 030 caches also have strange performance penalties - there's a small overhead for being 'on' and then there's a larger overhead for what it is doing while it is on. So you need to make sure they are effective when on.
One thing you do need to be careful of - if you turn the PMMU off and continue using the CPU caches, there's a risk you will be getting data cache hits on *some* hardware register areas. The TOS PMMU tree does mark HW areas as uncached and this will be ignored if you turn it off. There should really be bus lines to do this as well in hardware, but I'm not sure if they are connected up. I saw some serious flakyness with the PMMU off and d-cache on, so make sure you understand the effects of what you are doing before you use it :)
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