I enabled ECC after booting Linux on Athlon64

Hi all,

Until now, I have thought that it is almost impossible to
enable ECC functionality after the BIOS has booted the
computer. Many people in the ECC mailing list have
confirmed that, and I have not known of anyone even
attempting that seriously. The problem is that all of
memory has to be written to with correct checksum mode
enabled before the checking and correction of errors can be
enabled. This is usually done in BIOS, as there are no
other processes running.

Today, 2005-06-21, I noticed however that using the
Athlon64 hardware scrubbing feature allows the memory to be
initialized completely before enabling the checking of
memory reads. The scripts below have worked for me, and I
have verified that the correct ECC modes are enabled (with
ecc.pl), and that single bit errors introduced by grounding
memory data leads result in correct reports in CPU
North bridge registers.

My motherboard is Asus A8N-SLI Deluxe, which would be a
perfect computational node building block if the ECC was
correctly initialized in BIOS. It is not. Maybe this helps.

These scripts are at http://www.iki.fi/hyvatti/sw/ :

* ecc-start.sh enables the Athlon64 and Opteron 64+8 bit
ECC in runtime.

* ecc-chip-kill-start.sh enables Athlon64 and Opteron
128+16 bit Chip-Kill ECC in runtime. You must have a
128 bit memory configuration for this to work.

* ecc-stop.sh Stops the ECC operations.

* ecc.pl reads AMD64 CPU integrated north bridge registers
and displays them with names and descriptions bit by
bit. Latest version 2005-06-21 includes tables for ECC
syndrome codes. Their usage in decoding registers is
however unimplemented.

Testing the ECC functionality can be done with following
steps:

1. Use ecc-start.sh or ecc-chip-kill-start.sh.

2. Run ecc.pl to see that ECC is enabled and that the
syndrome registers are zero.

3. Short circuit a data line or preferably a parity bit
data line on one of the DDR memory modules with ground
for a short period. For example pin 49 (parity bit 2,
ie. bit 66) and pin 51 (parity bit 3 = bit 67) are
fine. The pin between them is Ground. Count the pins
from a DIMM where number 1 pin is marked. It is easy to
stuff a lead to the DIMM socket into the holes next to
the socket pins. I used 10 ohm resistor to make the
probability of damage smaller. However I am not
responsible for any damage you cause for your computer.
It can break completely.

4. Run ecc.pl to see if the syndrome codes are there. If
not, repeat the previous step.

5. Decode the syndrome code with AMD document 26094.PDF or
the table in ecc.pl. See if it corresponds with the
data lead you just shorted.


I hope this information is of use to at least some bluesmoke and ecc
users. Please let me know if you find out anything more on this subject
or can verify my results. Maybe this code can be included in the
bluesmoke distribution in some form.

Regards,
Jaakko