IBM 5160 oddball fault

[Jules Richardson]

On 04/23/2015 11:20 AM, tony duell wrote:
>>
>> Does anyone have any ideas what might be going on? It doesn't quite seem
>> like a memory fault - possibly some sort of address decoding error? It's
>> almost like the video board is pulling display data from the wrong part of
>> memory, but I'm not sure that makes sense given that the board has its own
>> local RAM rather than relying on RAM on the system board.
>>
>
> To me this sounds like an address line fault on the expansion bus. The CPU thinks
> it's writing to a particular location in the CGA card's memory, but due to the fault it
> actually accesses another location.

Yes, could be. CGA base address is B8000h, I think, and I'm seeing display 
text offset by 800h (2KB - 1024 characters plus 1024 attribute bytes). 
Assuming that the BIOS is responsible for clearing video memory (rather 
than it being a function of the CGA card itself), then my on-screen garbage 
could be down to whatever happens to be in the first 2KB of the CGA card's 
memory at power-on, and address bit A11 is jammed high on the expansion bus.

What's puzzling if that's the case is why the ROM BASIC 'function key' 
strip appears normally at the bottom of the screen - I'd expect it to be 
stuck in a 'non visible' portion of the CGA card's memory.

> Have you tried the CGA card in different slots? Just in case it's a bad contact
> on the edge connector.

Yes - the same thought had occurred to me.

I'll take a look at the address lines (albeit just with a DMM, but that 
will hopefully be enough if it's a stuck bit)

cheers

Jules