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Ensjo Offline OP
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Hello, all!

CoCo's Video Display Generator MC6847's specification says that the background of "black-and-green" Resolution Graphics 6 mode (accessible in CoCo via PMODE 4: SCREEN 1,0) is BLACK — and so it is implemented in M.E.S.S.

But in this video (taken from a real, non emulated CCE MC-1000, another microcomputer that uses MC6847), where the two palettes (black-and-green and black-and-white) are alternated, we perceive a slight change in background color:

http://www.ensjo.net/mc-1000/MVI_1261.AVI

The background of "black-and-green" Resolution Graphics 6 is not fully black — it's greenish — it's the DARK GREEN color we see as background for inverse text.

See this other video where RG6 and inverse text mode are alternated... and now we perceive no change in background color:

http://www.ensjo.net/mc-1000/MVI_1262.AVI

And indeed both effects you can get in CoCo Java emulator Mocha, running these programs:
Code:
10 PMODE 4 : PCLS
20 SCREEN 1,0 : REM BLACK&GREEN RG6
30 FOR A = 1 TO 500 : NEXT A
40 SCREEN 1,1 : REM BLACK&WHITE RG6
50 FOR A = 1 TO 500 : NEXT A
60 GOTO 20

Code:
10 CLS
20 FOR A = &H400 TO &H5FF : POKE A,32 : NEXT A
30 PMODE 4 : PCLS
40 SCREEN 1,0 : REM BLACK&GREEN RG6
50 FOR A = 1 TO 500 : NEXT A
60 SCREEN 0 : REM INVERSE TEXT
70 FOR A = 1 TO 500 : NEXT A
80 GOTO 40

It seems then that, despite what the specification says, MC6847's implementation in M.E.S.S. needs to be changed: black-and-green RG6's "black" is DARK GREEN instead of BLACK.

Can anyone confirm that that's how it looks in real, non emulated CoCos?

Last edited by Ensjo; 09/04/09 06:15 PM.
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On a CoCo3, both screens show a black that is as pitch black as black can be. I have heard that on some older CoCos, the displayed color is a very dark green instead of black. I do not remember for certain, though I think this to be the case. I have not taken my CoCo1 off it's shelf yet to verify this though...

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you sure the brightness/contrast isn't set too high?

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Originally Posted By Donny
you sure the brightness/contrast isn't set too high?

Yes, I'm sure. But as I said, some older CoCo behave as you described if I recall correctly.

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Ensjo Offline OP
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Originally Posted By Barry Nelson
On a CoCo3, both screens show a black that is as pitch black as black can be. I have heard that on some older CoCos, the displayed color is a very dark green instead of black.

According to Wikipedia, CoCo 3's video hardware differs from CoCo 1 and 2's, while keeping great compatibility. Maybe CoCo 3's real black in black-and-green RG6 is among the changes.
Originally Posted By Barry Nelson
I have not taken my CoCo1 off it's shelf yet to verify this though...

Could you please, Barry? Or anyone? smile


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