AMD 64 MB Features ?

[phil2004]

I`m considering getting a new PC and, since I`m not a hardware expert, have a few questions about Athlon 64 motherboards - sorry if they sound dumb -

1.) Why is it that the motherboards all seem to be limited to 3 gigabytes of RAM ? What`s the point of massively increasing the address range but providing less RAM than a 32-bit system can handle ? Are the new socket boards going to address this issue (higher end Opteron boards already do) ? An 8 gigabyte board would be nice .

2.) Does anybody know whether the new chipsets from nvidia and VIA will incorporate PCI/AGP locking ? Release timescales ?

3.) Linux drivers - which board/s have the best supported chipsets (nvidia ?) ?

If there are going to be improvements in these areas I may be prepared to wait before making a purchase. The RAM issue is of most interest for me as I intend to run multiple virtual machines - I could solve this by buying an Opteron system but I just can`t afford anything like that at the moment (unless anyone wants to point me in the direction of some cheapish hardware).

As I said, I`m no expert, so if anybody has any strong feelings about the current crop of Athlon 64 boards feel free to comment.

Thanks in advance.

EDIT:

I forgot to add - I`ve heard that the RAM may not always run at the highest speed on some boards, sorry I can`t remember the exact source, or reason why ?

 

[Peter]

In 32-bit environment, you have 4 GB of total address space. BIOS ROM, PCI devices, interrupt controllers and all the other stuff got to live somewhere, and then there's some empty space needed for virtual memory management. That's why you cannot reasonably expect to put more than 3 GBytes into a machine that's running a 32-bit operating system.

Now AMD64 has a 40-bit address space, allowing much more RAM. You'll need a 64-bit enabled operating system to make use of that. Note that it doesn't matter whether it's an 8-way Opteron system or a single Athlon-64. It's just that with each CPU, you get another RAM controller as well, adding to the number of DIMM slots the system can potentially have.

And then there's the problem that DIMMs currently are no larger than 1GB, so with 3 DIMM sockets, what'd you do to put more in?

 

[phil2004]

Thanks for the reply Peter.

I understand the limitations if you are going to be using the processor in legacy 32-bit mode - which I won`t be for that long hopefully. But, for example, here is a Tyan single Opteron board that provides for up to 8 gigabytes of DDR memory using four slots (rather than the three found on the average Athlon board - standard ATX form factor) - http://www.tyan.com/products/html/tomcatk8s.html . So if Tyan can do it why can`t everyone else ?

 

[Peter]

That's because the Opterons have twin memory channels, and run them in Registered mode, which effectively doubles the maximum feasible size of each individual DIMM by allowing twice as many chips on.
Now dual channel RAM and the requirement of Registered DIMMs substantially drives cost up - that's why you're not getting it on a consumer grade mainboard. If you serious performance, you need to get serious about everything - RAM technology, expansion busses, storage controllers and media. Toy mainboards w/ unbuffered RAM and 32-bit 33 MHz PCI are not going to cut it. That particular Tomcat board is halfway there - workstation grade processor, but no fast I/O bus. Look at the 2885 "Tiger K8W" for something that goes all the way.

Still, if you use such a big gun mainboard and actually stuff that much RAM on, you're not going to get to use it with a 32-bit OS.

 

[phil2004]

Right... so would that mean that upcoming socket 939 technology with a dual channel memory controller (unregistered RAM) will/could provide something similar to the Opteron boards ?

 

[Peter]

socket 939 gets you dual channel unbuffered RAM, plus single processor-only architecture. That'll get you four slots at full speed (DDR400), with 16 chips max per DIMM. With current RAM technology, that'll get you 4 GB of RAM, eight once the 1-Gbit chips enter mass production.

 

[nippyjun]

Which future chipsets will be supporting socket 939? Will the Nforce 3 250 chipset do so?

Also, will the mobo's that support 939 also support the current chips?

 

[Peter]

Mainboards obviously need to be specific. For a chipset to support socket-939, the HyperTransport link must be capable of 250 MHz operating frequency (that's "2 GHz" in marketingspeak, because of DDR mode and separate 16-bit up- and downlinks).