[Coco] coco2 MC6883/6847 memory map

[Arthur Flexser]

P.S.  The docs, which are here:

http://www.colorcomputerarchive.com/coco/Documents/Manuals/Applications/Key264K%20(Key%20Color%20Software).pdf

have a copyright date of 1983, quite a bit before the recent estimates
here of when CoCo multitasking began.

The program is here:
http://www.mmnt.net/db/0/0/ftp.maltedmedia.com/coco/SOFTWARE/DISK/Applications/Key-264K

Art

On Mon, Nov 10, 2014 at 4:47 PM, Arthur Flexser <flexser at fiu.edu> wrote:

> 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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>> Coco at maltedmedia.com
>> https://pairlist5.pair.net/mailman/listinfo/coco
>>
>
>