dmidecode weird total/data width

Solution 1:

You're right that you should be seeing 72 bits in that output, as DJ Bernstein describes, and as I quote from this concise answer on StackExchange:

• Data width = 64 (8 banks * 8 bits)
• Total width = 72 (9 banks * 8 bits)
• The extra bank indicates that ECC is active.

Your problem is almost definitely a BIOS bug. For my ASRock E3C224D2I, here's what dmidecode -t memory | grep Total shows for an old BIOS (P2.00) which incorrectly reports the data:

Total Width: 128 bits

If I update the BIOS to P3.50 I get:

Total Width: 72 bits

There's a thread on the FreeNAS board which alludes to this, but I haven't been able to figure out what the underlying detailed rationale for the bug is.