I tried a pair of 2GB DDR400 Reg ECC 128Mx4 modules in an Asus A8N-SLI. These are the same modules that work in a socket 940 server (where I took them from). It's only a partial success. They work, but for some reason the BIOS only makes 3GB usable.
The POST screen and the setup screen show that 4096MB are installed but 3072MB "usable". A Linux Mint 17.3 64bit USB flash drive could only access 3GB of the RAM. This drive also has memtest86+ 4.20 on it, and that also only sees 3GB.
I think I've used that flash drive before with machines that had larger amounts of RAM, so I don't think it's an OS limitation. It seems that the BIOS really is limiting the usable RAM to 3GB, and it basically says so on the POST screen.
There is a setting in the BIOS for "S/W DRAM Over 4G Remapping" which was enabled by default. I tried disabling it but that reduces the usable RAM down to only 2.5GB. With that option Enabled, the usable RAM is 3GB.
I tried adding a 3rd module, a 1GB registered DDR333, so now 5GB was installed. This actually caused the BIOS to only detect 2GB installed and usable. Then I added a 4th module, a 512MB registered DDR333 - making a total of 5.5GB of registered memory installed. At this point the board no longer POSTs.
The board is running BIOS 1014, which according to Asus' support page is the last official release. They do have a beta BIOS from about 1.5yrs later which I have not tried. There is no notation of what the changes were in that beta BIOS.
Bottom line, it seems to be impossible to exceed 3GB in the A8N-SLI. Asus advertised support for 4GB but in reality it seems their BIOS limits it to 3GB. Granted, Asus did not officially endorse registered modules, but somebody here:
https://forums.geforce.com/default/topic/3561 … deluxe-how-to-/
apparently had the same problem and I would assume they were using the officially endorsed unbuffered memory that a consumer would have been buying in 2006. The only response to that person points to an FAQ entry which is 404.
If anybody has had more than 3GB recognized on any version of the A8N-SLI (non-32) boards, please describe how you did it.
Perhaps a different BIOS will do something. I might try the beta BIOS.
The one thing I can say is using ecc-reg ram in the A8n32-SLI series boards you have to have memory hole enabled in bios, and ECC enabled or it won't POST/Run with high density ecc-reg dimms (2GB and higher). I remember having to use a couple sticks of 1GB sticks in there to enable those settings, then shut down, install the 4GB sticks then it came on.
There's an ECC section for the ram settings in bios, that normally is not displayed with "normal desktop" ram, but when the system detects ecc ram, and you boot with it and go back in to bios, it shows up. At least that's how mine worked, with the latest bios version.
On my A8N-SLI (BIOS 1014, non Premium, not the "32", just plain A8N-SLI) the behavior is similar. For me, if "ECC Master Enable" is disabled, then it would not POST with even a 1GB DDR333 Reg ECC module. It did however POST with a 512MB DDR333 RegECC. It would also POST with 512MB DDR400 unbuffered ECC. Either of those 512MB modules was sufficient to make the ECC toggle available, and once that was enabled, then the 1GB and 2GB regECC modules began working, but with the odd 3GB "usable" limitation described above.
I must say I've found this board very quirky in how it responds to various arrangements of RAM. As mentioned above, adding a 3rd and 4th regECC module caused memory size to *drop* and then no POST altogether, respectively. Using a single module in the wrong slot also causes no POST. If ECC is disabled, no POST with larger modules (as kithylin described). I don't remember when I've used a board that was so glitchy about what combinations are installed and where. It seems like the BIOS has a half broken ability to sync up SPD information and gets confused easily.