JannievanZyl wrote:Meant, the exact bit pattern that would show the problem. Was hoping all the hints the system throw out would have pointed to a specific bit or range of addresses.
If the problem is a failure with only one DRAM chip, it will affect only a single bit, but typically affect many (if not all) addresses within that chips memory.
If the problem is that of an addressing error (the addressing of RAM in a BBC Micro is rather complex), then either the data is being written to more than one address, or maybe being read from more than one address and two chips are fighting one another's outputs.
As you now have problems with MODE 7 (and it's only 1k bytes of data), you could compare what the CPU reads from this memory area when you have corruption on screen. This will tell you for certain if the problem is a faulty DRAM chip, or a problem with the data being read by the display system/addressing of the DRAM by the display system.
The other thing to keep in mind, is that it is the display system that provides DRAM refresh. It does this by continually reading the DRAM even if the data is not needed for the display (/RAS and /CAS active for the addressed DRAM chip). As it scans through the addresses needed for the display, enough DRAM chip row addresses are scanned (/RAS goes active, but /CAS does not) so that ALL the DRAM chips will refresh the cells in that row. Also, in a DRAM chip, every time a memory cell is read, the stored value is destroyed, so circuitry in the chip has to restore the cell contents. It's possible that there is a fault in one of the DRAM chips where sometimes this data restoration is not working correctly, so as data is read by the display system, the corruption spreads...
So as you can see, identification of the cause from the symptoms is sometimes not so easy...