[Coco] [Color Computer] updated wikipedia article
jdaggett at gate.net
jdaggett at gate.net
Fri Nov 18 13:34:28 EST 2005
Art
The Linear Address counter inside the SAM chip is not identical to that inside the
VDG chip. The counter inside the SAM chip is more complex and allways the
semigrahics modes 8,12 and 24. By using the address generator in the SAM chip,
one gains more flexability than using the address counter inside the VDG.
Conversely the Linear Address counter inside the GIME is different from the SAM
chip. That is why the GIME chip can not do semigraphics modes 8, 12 and 24.
There is a common misconception among some in the Coco community that the
VDG and the SAM chip must be used in conjuction with each other. That is not so.
Each chip will work with any 8 bit processor without the other. With both you get the
full capbilities that both chips provide. The Coco 1/2 can do what they do only
because Tandy used both chips.
james
On 17 Nov 2005 at 19:07, Arthur Flexser wrote:
Date sent: Thu, 17 Nov 2005 19:07:39 -0500 (EST)
From: Arthur Flexser <flexser at fiu.edu>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Subject: Re: [Coco] [Color Computer] updated wikipedia article
Send reply to: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
<mailto:coco-request at maltedmedia.com?subject=unsubscribe>
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> Perhaps I misinterpreted what exactly was the glaring error. Are you
> saying that the writer was mistaken not simply in claiming that the
> SAM has 12 address bits when instead it has 16, but was more broadly
> mistaken in characterizing there as being 12 bits in the SAM that
> correspond in the usual display modes to 12 analogous bits in the VDG?
> I had not previously heard that claim, but assumed it to be correct
> since it is certainly the case that certain VDG and SAM input values
> "go together" in producing the common display modes, and that you get
> unusual display modes by mixing the values in other than the common
> combinations.
>
> Art
>
> On Thu, 17 Nov 2005 jdaggett at gate.net wrote:
>
> > Art
> >
> > To me it appears to point someone unknowing of the Coco Internals as
> > thinking the two are near identical. At least that is my perception.
> > Then I maybe out of bounds from the rest of the group.
> >
> > james
> >
> > On 17 Nov 2005 at 15:07, Arthur Flexser wrote:
> >
> > Date sent: Thu, 17 Nov 2005 15:07:37 -0500 (EST)
> > From: Arthur Flexser <flexser at fiu.edu>
> > To: CoCoList for Color Computer Enthusiasts
> > <coco at maltedmedia.com> Subject: Re: [Coco] [Color Computer]
> > updated wikipedia article Send reply to: CoCoList for Color
> > Computer Enthusiasts <coco at maltedmedia.com>
> > <mailto:coco-request at maltedmedia.com?subject=unsubscribe>
> > <mailto:coco-request at maltedmedia.com?subject=subscribe>
> >
> > > I'm not really seeing the "glaring" error. To say that the SAM
> > > contains a duplicate of the VDG's 12 bit address counter does not
> > > deny that the SAM might have more address bits available as well,
> > > just that 12 of its bits would normally duplicate the VDG's 12
> > > bits.
> > >
> > > Art
> > >
> > > On Thu, 17 Nov 2005 jdaggett at gate.net wrote:
> > >
> > > > Mike
> > > >
> > > > One glaring error that I see is in the following paragraph:
> > > >
> > > > Due to limitations in 40 pin packaging, the SAM contains a
> > > > duplicate of the VDG's internal 12 bit address counter. Normally
> > > > this counter's settings are set to duplicate the VDG's display
> > > > mode. However this is not required and results in the creation
> > > > of some new display modes not possible when the VDG is used in a
> > > > system alone.
> > > >
> > > > The linear address counter for video memory in the SAM chip is a
> > > > 16 bit counter and not a 12 bit counter. Setting the F0-F6 flip
> > > > flops allows the video ram to be set on any 512 byte boundary
> > > > within the 64K memory map. This difers from the MC6847. The VDG
> > > > supplies the lower 12 bits of address and has to rely on
> > > > external address decode logic to be able to map where video ram
> > > > was located in 8K boundaries. Due to the increase capabilites of
> > > > the SAM chip's linear address generator for video memory, the
> > > > SAM chip inconjucntion with the VDG allowed increase of three
> > > > semi graphics video modes.
> > > >
> > > > Also someone unfamiliar with the Coco line would think that the
> > > > MC6883/74LS783 and the MC6847 were developed exclusively for the
> > > > Coco. This inference would not be true. Each chip can operate
> > > > with any 8 bit computer on its own and not with exclusively with
> > > > the MC6809/MC6809E. Furthermore the MC6883 and the 74LS783 are
> > > > the same chip. The SAM chip is LSTTL logic and was done on a
> > > > bipolar line as opposed to the VDG chip which is High Density
> > > > NMOS and done on a MOS line. I am not sure that this is the
> > > > "gospel" as to how it got two part numbers but one possibility
> > > > is related to which camp the part went to. If sold under the
> > > > LSTTL catalog it would have the 74LS783. Conversely if sold
> > > > under the Motorola Computer Products it had the MC6883 label. I
> > > > vaguely remember around the 1980 time frame that there was a
> > > > growing number of parts that carried dual numbers and there was
> > > > a cleanup or consoldation of internal/external documentation.
> > > >
> > > > Finally the ACVC or GIME is woefully breif if one is trying to
> > > > give a technical description of the hardware. The GIME chip is
> > > > not 100% backwards compatible. It is not just an integration of
> > > > the MC6883 and the MC6847. It is by far a more complex IC than
> > > > the previous pair and a design of its own from the ground up.
> > > >
> > > > james
> > > >
> > > > On 16 Nov 2005 at 19:52, Mike Pepe wrote:
> > > >
> > > > To: ColorComputer at yahoogroups.com
> > > > From: Mike Pepe <lamune at doki-doki.net>
> > > > Date sent: Wed, 16 Nov 2005 19:52:32 -0500
> > > > Copies to: Subject: [Coco] [Color Computer]
> > > > updated wikipedia article Send reply to: CoCoList for Color
> > > > Computer Enthusiasts <coco at maltedmedia.com>
> > > > <mailto:coco-request at maltedmedia.com?subject=unsubscribe>
> > > > <mailto:coco-request at maltedmedia.com?subject=subscribe>
> > > >
> > > > > Hi listers,
> > > > >
> > > > > I was working on a "CoCo Tech" web page, but when I saw the
> > > > > entry for the Atari 8 bit machines on wikipedia I decided to
> > > > > integrate my work there instead.
> > > > >
> > > > > http://en.wikipedia.org/wiki/TRS-80_Color_Computer
> > > > >
> > > > > now includes a lot of hardware information, though I'm not
> > > > > sure if that would really be of interest to most people.
> > > > >
> > > > > Comments are welcome.
> > > > >
> > > > > -Mike
> > > > >
> > > > >
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