23 Apr
2015
23 Apr
'15
10:58 a.m.
On 4/23/2015 10:30 AM, tony duell wrote:
The cursor actually has nothing to do with where the
data is written or
appears. And I understood the cursor appeard correctly, that is in the
location you would expect characters to be written, even though they
are not. This suggests the 6845 is being set up correctly but that the
processor is not writing to video memory where it thinks it is.
I guess it's also possible that the 6845 is not being set up correctly
to start the display at the first location of video memory. The problem
then is that the 6845_is_ being set up correctly to generate valid timing,
etc.
That was what i suggested. it looks like the video memory, and infact
most all of the memory operations are working or basic would not come
up, or the bios codes for memory errors would occur.
So the mechanism which sets up the 6845 is broken, with the skew of the
location of the first message being significant, if it is a multiple of
perhaps one or more bits stuck when the 6845 registers are written.
I suspect the setup of the addressing of where the display memory is not
relevant, so once you mess up where in the memory to fetch from, all the
memory accesses are skewed but still reference the data the processor is
writing to that memory (which appears to be correct).
since there is a lot of crap at the top which is not set up correctly,
perhaps there is a skew factor to be guessed at from that number of
lines or characters.