At 03:35 PM 3/29/99 +0200, shevek wrote:
>> *The size of a memorymapper is at least 64kB (four pages of 16kB)
>
>I don't agree with this. Although all mappers are at least 64kB, I don't
>see any reason for demanding it to be.
Good point. Jon only needed the minimum size because he wanted to read the
ports and use the lower 2 bits.
>I think we should not even define that it must have a
>number of pages which is a power of 2. It may happen somebody wants to
>make a mapper of 48kB or something as a test.
Gouda offered mappers of 640K at some time. Combined with 128K VRAM that is
enough memory for FastCopy to read an entire disk at once.
>> -Since all valid memory mappers contains at least 4 mapperblocks, at
>> least 2 bits are used for the mapperports. When writing data to a
>> mapperport, the two least significant bits can always be read back.
>> (useful to check if there is a memory mapper present in the system)
>
>Is it useful to know if there is a mapper if you don't know where it is?
No, it isn't. ("where" = "slot")
To check the presence of the mapper, you don't have to read the mapper
ports. Just like you can detect a megaRAM, although the megaRAM mapper
cannot be read.
On any slot containing RAM, you use the "count pages" algorithm. If you get
"1" as a result, it is either no mapper or a 1-page mapper (makes no
difference in use). If you get a higher number as result, there is a mapper
in that slot.
Bye,
Maarten
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