AM2 motherboard memory limits

[bullfrawg]

If AM2 CPUs have the memory controllers onboard, then why do different AM2 motherboards list different limits for maximum memory? For example, the MSI nForce 550 motherboard lists 4 MB, while the MSI nForce 570 SLI board lists 8MB, and the Gigabyte nForce 570 SLI board lists 16 GB. (Chipsets, not model numbers.)

I am concerned because I think that a main-memory limit of 4 MB will make a motherboard obsolete sooner than the AM-2 socket, the PCI-e x16 graphics slot standard, the SATA-II hard disk standard, or anything else on the motherboard that can't be corrected with an add-in card. And it seems to me that when you have to replace the motherboard, you are likely to end up replacing just about everything in the computer. I'd like to have a computer that can be continuously upgraded for a long time before I have to start over.

 

[eplebnista]

curiosity bump

 

[Noubourne]

I'm not an engineer or anything but I'll have a crack at this.

The boards have to implement a certain amount of bandwidth over the hypertransport bus for the RAM. These are physical channels of metal on the board over which the data will be sent. The more you provide, the more complicated the design of the board, which means it is more expensive to engineer and manufacture. Thus, the cheaper boards implement fewer channels and therefore have less bandwidth and so are capable of handling less total RAM.

 

[Operandi]

The Athlon64 CPU dose have direct access to the RAM but I'm fairly certain that the chipset dose as well so it would still be up to the board and chipset manufactures to determine final capacity and timing specifications.

 

[bullfrawg]

@Noubourne: But some of the cheapest ECS and PC Chips boards on newegg list 32GB or 16 GB, while the DFI and Biostar nForce 590 boards list 4 GB -- and only Gigabyte offers an nForce 590 with more than 8 GB. It should only take one additional metal channel to double the amount of memory that can be addressed. Still, that could be it. Everyone has to cut corners somewhere, and maybe DFI expects its customers to get a newer bleeding-edge board before 4 GB becomes a factor, while ECS customers want to feel that they can hang onto that motherboard for the long haul and add lots of memory? I've noticed that max memory is more correlated to the manufacturer than to board price: on AM2 motherboards, Epox and Gigabyte usually allow 16 GB; Asus, MSI, and Abit usually allow 8 GB; and DFI and Biostar usually allow 4 GB.

@Operandi: I think I read that this has been one of AMD's problems with respect to mobile computing: the on-chip memory controller made it difficult for integrated graphics to share memory. So I think the chipsets can't directly access the RAM. I could certainly be wrong.