Re: HGR Displays During HBL?

["Michael J. Mahon" <mjmahon@aol.com>]

Michael J. Mahon wrote:
> Bryan Parkoff wrote:
>
> >     You may notice magenta (deep red) color appears in the screen 
> > during HBL.  You can type below like this in Applesoft BASIC prompt.
> >
> > ]HGR
> > ]HCOLOR=5: HPLOT 279,0 TO 279,191
> > ]HCOLOR=4: HPLOT 0,0 TO 0,191
> >
> >     Do you notice that orange vertical line becomes brown in the far 
> > right? I know that it is cut-off, but did you see magenta vertical 
> > line in the far left?  If you fill 0x40 in memory address: 0xXXFF, it 
> > will fill magenta vertical line from line 0 through the middle line.
> >     It has a bug on Apple II+, but Apple //e has fixed the bug.  Can 
> > you please explain why Apple II+ has reproduced bug for curiousity?  
> > 0xXXFF can have data, but they are not supposed to be displayed during 
> > HBL which it must always be undisplayed.  It is how magenta vertical 
> > line detects 6th bit of data bus which it is turned on to form magenta 
> > vertical line while vertical line in position 0 is always filled with 
> > 0x80 (delayed black).
> >     Is it possible that the designer made mistake to design Apple 
> > II+'s motherboard with TTL chip?
>
>
> It's just an artifact of the horizontal blanking starting and
> ending a little earlier than would have been perfect.  NBD.

It is a reflection of the fact that the "second set" of delayed
colors was an afterthought, not taken into consideration in the
timing design of horizontal blanking.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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