IBM 5160 oddball fault

jwsmobile jws at
Thu Apr 23 12:58:10 CDT 2015

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.

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