RE: strange cursor... any idea ?

Hallo Didier,


The interrupt routine only inverts bit 7 at $e476. My question is: do you see a checker board all the time? I think answer is "yes" because the shown checker board is $7F. And I assume that 1) the checker board shows up when you move the cursor over an existing character and 2) that this character reappears when you move the cursor? Added later: does it "blink"?

If so, here are my thoughts (so far):
- This is a software cursor, the hardware cursor of the 6845 is not used, so we can rule out the 6845 itself.
- The character is read at $E46e, "lda    (CurScrLine),Y"/"B1 C4". This is a two-bytes instruction. If this instructio is replaced by "lda #$FF"/"A9 FF", then you have a possible answer for your problem. Problem here: this answer does change the contents of the screen memory but does not restore the original value when you move the cursor. Just check if your E-ROM is OK and that should rule out this answer.
- Broken CPU? No, because: what could be broken that it only shows up now?
- Broken RAMs? No, that would show up in other characters as well. And you did replace that.
- Broken buffers between the video RAM and CPU, in the direction form RAM to CPU? No, then all four ICs have to be broken.
- Bad signal that steers the mentioned buffers? That could be but: if the buffers don't output anything, $FF is read. Inverting bit 7 means it becomes $7F. Question, you didn't mention that the cursor "blinks" so: does it? If not, that explains a steady checker board. The bummer of this answer is the same as above: it does change the contents of the screen memory but does not restore the original value when you move the cursor.

So the question remains: what does change the read value into $7F but does not change the contents of the video memory? No solution from me but maybe it triggers someone else to have another idea.


Met vriendelijke groet,

Ing. Ruud Baltissen
Senior Beheerder TI

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