[Coco] coco2 MC6883/6847 memory map

[Tormod Volden]

On Mon, Nov 10, 2014 at 2:39 PM, 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
>

The SAM (6883/74LS783) has a memory map type /and/ a RAM page switch.
Brett is talking about the memory map type which basically switches
between RAM+ROM (map 0) or all-RAM (map 1).

The page switch decides which of the 32K RAM banks is accessed in map 0.

This was used by Sarah from the Dragon forums for her "Double Dragon"
- multitasking two (32K mode) BASIC programs/environments on a Dragon
64: http://archive.worldofdragon.org/phpBB3/viewtopic.php?f=5&t=488&p=2620#p2620

> On Mon, Nov 10, 2014 at 7:53 AM, didier derny wrote:
>> I'm trying to understand how the coco 2 works J
>>
>>
>>
>> It's relatively clear except for one thing
>>
>> [I probably missed something in the datasheets]
>>
>>
>>
>> When a coco2 is running with the basic I suppose it is in map type 0
>>
>> With 32k rom/io  and 2 pages 32k ram  (if 64k installed).
>>
>>
>>
>> If the processor is in page 0  where is the video ? page 0 or page 1 ?

The processor is happily unaware about the page switching. It is the
SAM that is in page 0 or 1. The video ram access goes through the SAM,
to the active page.

Tormod

>>
>>
>>
>> And what happens if I switch from page 0 to page 1
>>
>> The video also switch  ?