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Atari coding BBS
| Re: PMMU question |
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Posted by: dml
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Mar,14.2014-13:08
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Actually a quick google produced this, from Pink/RG:
http://mikro.naprvyraz.sk/docs/Coding/Atari/Maggie/MMU.TXT
...on Mikro's site no less! :-p
"Fortunately the MMU has a cache to store table entries - this is known
as the Addres Translation Cache (ATC), and it works in a similar way
to the data cache. When the MMU has to perform a translation, it first
searches its ATC to see if the entry is already included. If its not
there, it has to be fetched from main memory.
This will have some affect on performance. The first access to page
will cause the MMU to fetch an entry from the table, but subsequent
accesses to that page will be fast. The ATC only has 22 entries.
So to optimise for the ATC, ensure you perform as many accesses to the
same page in the same code block - try not to jump all around memory.
Every time a new page is encountered, performance will suffer for the
first access.
The 68030 always uses the MMU. TOS intialises it so all your code is
running through the MMU. This may seem frustrating if you are just
addressing memory as normally and don't won't the functionality of the
MMU. You are not getting its benefits, so why should you get its
performance hits?
Fortunately the MMU caters for this case. It has two registers for
Transparent Translatioin. These identify blocks of memory that can be
translated without access to MMU. Unfortunately, the slice of memory
that the Transparent Translation registers can specify is 16MB."
There's more detail on the page itself, worth a read.
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