- Joined: 28 Sep 1999
- Posts: 1197
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Paging behavior regarding battery backed RAM.
 Posted: Tue Sep 28, 1999 5:28 am
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Here's an imaginary situation I'm trying to work out:
If a game sets 8000-BFFF to (for instance) ROM page 3, then
enables RAM in that address space, then disables RAM, should
any following reads still be from ROM page 3, or would it
'reset' to page zero?
And if you enable RAM in that area, then write to FFFF to change
the ROM page, then disable RAM, would the value written take
effect, or would the last value written before RAM was initially
enabled be used?
I don't know of any games that actually work like this, but I'd
love to get some ideas on what would happen. :)
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Eric
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Posted: Tue Sep 28, 1999 4:30 pm
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The following is my understanding, which has NOT been tested on the actual hardware. It is based soley on my understanding of various SMS technical documents, as well as general knowledge of computer hardware implementations.
Quote > If a game sets 8000-BFFF to (for instance) ROM page 3, then
> enables RAM in that address space, then disables RAM, should
> any following reads still be from ROM page 3, or would it
> 'reset' to page zero?
Because the bit(s) controlling back-up RAM are in a separate register (at 0xFFFC) than the Page Frame 2 (addresses 0x8000-0xBFFF) paging register (at 0xFFFF), it seems that once back-up RAM is "disabled", the page frame will continue to map the previous ROM page.
To answer your question directly, the following reads would indeed read from ROM page 3 and not page 0.
Quote > And if you enable RAM in that area, then write to FFFF to change
> the ROM page, then disable RAM, would the value written take
> effect, or would the last value written before RAM was initially
> enabled be used?
Again, since the registers controlling back-up RAM and paging are separate, any write to one is independent of the others. It is likely the write to 0xFFFF would take affect, and the new ROM page would be mapped into the page frame once back-up RAM is disabled.
Quote > I don't know of any games that actually work like this, but I'd
> love to get some ideas on what would happen. :)
Eric Quinn
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