[Coco] coco2 MC6883/6847 memory map

[Arthur Flexser]

There was a program for the CoCo that sounds similar to the Dragon program
described.  I believe it was called KEY264K, and was advertised in
Rainbow.  You could have two Basic programs simultaneously, one in each of
the 32K RAM banks, and either switch between them or have both running at
once, with one running in the background.  I suppose that could be
considered multitasking, possibly before OS-9 came on the scene.  (Somebody
was asking when CoCo multitasking first appeared.)

Art

On Mon, Nov 10, 2014 at 9:50 AM, Tormod Volden <lists.tormod at gmail.com>
wrote:

> 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  ?
>
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