[Coco] coco2 MC6883/6847 memory map

[Brett Gordon]

Cool... I never knew that!  I guess I didn't study the SAM datasheet
very well.  Sorry if I led you astray Didier, my answer was only
half-right, at best!  Sarah's project looks cool btw.

On Mon, Nov 10, 2014 at 10:04 AM, Robert Gault <robert.gault at att.net> wrote:
> Brett Gordon wrote:
>>
>> Hey Didier,
>>
>> The coco2's video processor is logically separate from the memory
>> mapping.  When basic starts up it is in Page 0 mode... where the
>> bottom (lowest address) 32k is routed to ram, and the top 32k is
>> routed to the ROM (BASIC).  If you switch to page 1 the lower half of
>> RAM remains the same, but the upper 32k of address space is routed to
>> additional RAM (if it exists).  Video takes it's memory from anywhere
>> in the logical map, but BASIC setups this up at address 0x400, in
>> lower address space.  I found the use of the word "PAGE" for the
>> memory in the docs to be a misnomer.   And switch from Page 0 to Page
>> 1 doesn't do anything to Video... it still takes its memory from the
>> LOGICAL address space.  Just the Top 32k of the logical map is
>> switched out.
>>
>> -B
>>
>> <snip>
>
>
> Brett,
>
> That's not quite what the SN74LS783/MC6883 specs indicate. You have
> described page1 as equivalent to the map type1 All Ram mode.
>
> The normal state of a Coco2 is Map Type 0 + Page 0. That puts RAM from
> 0-$7FFF and ROM from $8000-$FFFF. Map Type 0 + Page 1 keeps ROM from
> $8000-$FFFF but swaps the lower RAM (if you have 64K RAM) from the lower
> half to the upper half. So, there are two switchable banks of 32K RAM.
> Map Type 1 ( all RAM mode) deactivates paging and the latch at $FFD4/$FFD5
> is inactive.
>
> Now given that video with a Coco2 can be set anywhere within the 64K range
> by the bytes $FFC6-$FFDF and is independent of the memory actually in the
> 64K range, flipping from page0 to page1 will alter the screen if the display
> offset is pointed to the first 16K of memory.
>
> Didier,
>
> You need to be more precise with your questions. There is a difference
> between video changing because the content changed or because the location
> of video in memory changed. Both of the aforementioned will change the
> screen content.
>
> Robert
>
>
>
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-- 
Brett M. Gordon,
beretta42 at gmail.com