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.