Re: HGR Displays During HBL?

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

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.

-michael

Music synthesis for 8-bit Apple II's!
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