Posted By
Bionic
on 2003-10-30
|  Re: Out-border text (graphics :) )
some additional info:
Why is it only possible to display charactor $00?
The character (and color) information is loaded into a 40x8bit memory on the TED during a bad line. During the character display two bytes of this memory have to be fetched for each char. To save some die space the chip designers did not use a real adress decoder, but a simple linear shift register. At a specific position to the left of the textscreen a single 1 is loaded into the shift register and shifted during each character cycle. After char 40 has been displayed, the 1 will be shifted out of the register so that no position of the memory is adressed and just some default value (apparantly $00) is fetched.
To display chars other than $00 the TED has to be tricked into resetting the shift register somewhere..
btw. here is a die-photo of the relevant parts of a VIC. This aspect is identical to the TED.
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