| Posted By
MCes
on 2022-12-19
08:52:46
 |  Question about a specific case of TED data bus access
On TED docs I can read:
When ROM is selected, the program residing in ROM appears in place of RAM.
The exception to this is a write operation to ROM will always ’bleed through’ to underlying RAM.
The question is:
if the external ROM is selected and I point the TED graphics addresses to the selected ROM,
will the TED fetch the graphics data from the ROM or the overlaid RAM? (or other?)
For example:
if I put 8K HIGH resolution data on ROM,
can these data be reached directly from TED or do I have to copy them to the corresponding ram addresses?
Thanks!
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Posted By
gerliczer
on 2022-12-19
10:17:05
| Re: Question about a specific case of TED data bus access
Please, refer to description of TED Register $12 ($FF12) Bit 2 e.g. in the Plus4World Encyclopedia. You may also want to check Evo Lution regarding an "actual" use case.
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Posted By
MCes
on 2022-12-20
04:53:26
| Re: Question about a specific case of TED data bus access
@gerliczer
Thanks, it's what I'm looking for!
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Posted By
TLC
on 2022-12-25
17:21:57
| Re: Question about a specific case of TED data bus access
@MCes the short answer is yes. The ROM mapping is independent to the TED's internal RAM/ROM mapping logic, and, when applicable, it is applied for both CPU and TED memory fetches. That is, if you tell the TED to fetch data from ROM, then it'll simply choose ROMs according to the ROM mapping register i.e. $fdd0-$fddf, and fetch data from there.
(A bit of warning, maybe not needed but anyway: while bitmask fetch is controlled by $ff12 bit 2, character pointer / graphic attribute memory fetch is not; that'd always follow $ff3e/$ff3f.)
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