[Coco] What do you make of this non-approved HSCREEN mode?

[theother_bob]

----- Original Message ----
From: "jdaggett at gate.net" <jdaggett at gate.net>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Sent: Tuesday, June 9, 2009 9:43:45 PM
Subject: Re: [Coco] What do you make of this non-approved HSCREEN mode?

On 9 Jun 2009 at 21:44, Robert Gault wrote:

> jdaggett at gate.net wrote:
> ><snip>
> > 
> > Robert
> > 
> > That is interesting find. Yes there are settings that become
> > meaningless. I do agree that they should have not been implemented
> > if it is not going to be supported or of use. 
> > 
> > james
> > 
> > 
> 
> Here are two other modes that don't behave as expected. They are the
> 160-byte and 128-byte 2 color modes. The hardware can't be expected to
> scan fast enough to yield 1280 or 1024 pixels and it is not clear
> exactly what it is doing.
> 
> The following program will demonstrate that these modes are one
> "pixel" per byte. Unfortunately that does not mean 256 colors. :)
> There is only one active bit determining the palette register, bit-7.
> If bit-7 is not set, the color is palette #0. If it is set the color
> is palette #1.
> 
> It might be more accurate to say that the pixel is not round but 8 by
> 1. Or put another way, all 8 pixels in a bytes are either color #0 or
> color #1.
> 
> 1  W=160:WW=28:              REM Or use W=128:WW=24
> 10 M=&H60000:PX=2^7          REM Use any other power of 2 as a test.
> 20 HSCREEN2:POKE&HFF99,WW 30 PALETTE1,25:POKE&HFFB0,7 40 FOR I=0TO
> W-1:LPOKE M+I,PX:M=M+W:NEXTI 50 GOTO 50
> 
> This may or may not add to our knowledge of the GIME circuitry. Who
> knows, if we completely characterize the unauthorized modes, we might
> find something exciting. :)
> 

>Robert 

>keep going. The best way to understand how the internals of a black box 
works is by tickling its inputs and looking at what the outputs do. I starting 
to digest what you have here. This is a first beginning to try and decipher 
the internal workings a bit more. 

>james



I've just stumbled onto the list for the first time in a while... best thread in quite a while.
Just wondering if this behavior is consistent with both '86 and '87 GIMEs.

My instinctive theory would be that it is just incomplete implementation. Maybe management put the rush on the design team to "finish up," so they had to ship it, no time to even close the doors or squash a bug. Time is money!

This may be totally irrelevant, but is the design loosely based on the 6847 architecture?
If you issue PMODE4,x and SCREEN1,x, then issue PMODE0,n without a SCREEN command, then draw, for example, concentric circles, the circles show up in two columns simultaneously. You issue one circle command and get two circles on screen, side by side. I'm just saying, it seems like sort of similar behavior.

Cheers,
Bob

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