general AMD64 question

[Bernoulli]

The major draw to 64-bit computing is the addressiblity of more system memory. With 40-bit memory registers, the AMD64-based processors should natively address 1TB of RAM.

Why, then, are the consumer (read: non-Opteron) boards limited to 4GB? Why don't they add support for 4 2GB DIMMs?





I suppose it may be because no one (to my knowledge) makes unbuffered 2GB DIMMs.


Thoughts?

 

[uOpt]

It is very difficult to connect RAM slots at full speed for the motherboard maker.

And BTW it is worse. Most, almost all AMD64 754 and 939 boards are not guaranteed to even fill the slots they have at full speed.

 

[DrCool]

I believe it is more of a current CHIPSET limitation then actual processor limitation.

Just like when the first generation of anything comes out, the technical papers are always much easier to write, then to actual develop technology from..

We could very well see new chipsets in the comming months / years that allow the same A64 processor to support 8GB natively.

 

[uOpt]

Well, mainboards with more RAM slots are available, they are just expensive and often unreliable, and often don't run the RAM at full speed.

The dual-Athlon at work has 6 GB and can take 8 IIRC but in all honesty I would have been happier with a straight 4 GB FX-55.

I didn't check what RAM speed it uses and I'm afraid to look

 

[Matthias99]

Originally posted by: DrCool
I believe it is more of a current CHIPSET limitation then actual processor limitation.

Just like when the first generation of anything comes out, the technical papers are always much easier to write, then to actual develop technology from..

We could very well see new chipsets in the comming months / years that allow the same A64 processor to support 8GB natively.

The memory controller is in the CPU on the Athlon64/Opteron. The chipset really has very little (if anything) to do with this.

The CPUs might actually support 2GB DIMMs if you could get unbuffered ones. However, I don't know if they can make them unbuffered and still fit the JEDEC specs for DDR RAM.

 

[DrCool]

My logic could very well be flawed, i'm not up-to-speed on the technical aspects on this.. but it seems the chipset should play some part, as it still has to address the memory in the board.

 

[Bernoulli]

Originally posted by: MartinCracauer
Well, mainboards with more RAM slots are available, they are just expensive and often unreliable, and often don't run the RAM at full speed.

The dual-Athlon at work has 6 GB and can take 8 IIRC but in all honesty I would have been happier with a straight 4 GB FX-55.

Well yah, most dual boards support 4GB per processor, but that 4GB came from the 32-bit memory registers (which are only able to address 4GB of memory).

Maybe I should have worded my question differently:

If they aren't going to make use of the memory potential, why did they go 64-bit in the first place?

 

[Matthias99]

Originally posted by: DrCool
My logic could very well be flawed, i'm not up-to-speed on the technical aspects on this.. but it seems the chipset should play some part, as it still has to address the memory in the board.

The memory is hooked directly to the CPU, because the memory controller is on-die. There's no "northbridge" chip on Athlon64 motherboards. All memory addressing is done through the CPU.

 

[Bernoulli]

I suppose it may not just be a question of hardware.

Lacking a main-stream 64-bit operating system (Windows splits memory to ~2GB per process anyway), there isn't really a point to having single processor machines with >4GB.

Perhaps with the release of Windows XP 64 and some applications we'll see larger capacity motherboards.

 

[Matthias99]

Originally posted by: Bernoulli

Originally posted by: MartinCracauer
Well, mainboards with more RAM slots are available, they are just expensive and often unreliable, and often don't run the RAM at full speed.

The dual-Athlon at work has 6 GB and can take 8 IIRC but in all honesty I would have been happier with a straight 4 GB FX-55.

Well yah, most dual boards support 4GB per processor, but that 4GB came from the 32-bit memory registers (which are only able to address 4GB of memory).

Maybe I should have worded my question differently:

If they aren't going to make use of the memory potential, why did they go 64-bit in the first place?

There have been many, MANY threads on this. There are more advantages than just having more RAM, such as being able to work natively with 64-bit integer operands, and having extra registers available in x86-64 mode. Also, you still have a >32-bit virtual address space, even if you have <= 4GB of physical RAM in the box.

Not that any of these things are being used unless you have a 64-bit OS.

 

[Bernoulli]

Originally posted by: Matthias99

There are more advantages than just having more RAM.

Agreed, but you wouldn't know it from advertising. Most every advertisement for 64-bit systems mentions something like "smash the 4GB barrier," etc..

Maybe that was just Apple.

Originally posted by: Matthias99

Not that any of these things are being used unless you have a 64-bit OS.

Correct, hence my previous post.

 

[uOpt]

Uh, not correct. Some people here don't understand the difference between paged virtual memory and physical RAM. It can make perfect sense to have more than 4 GB mappable space even if you have 128 MB of RAM.

For the same reason, it is not "4 GB per processor". With PAE you can have as much RAM as you want in a 32 bit architecture. The limitation is that each process (not processor) can only address 2, 3 or 4 GB at a time (depending on OS). And that is paged virtual memory and not pysical RAM.

 

[Viper96720]

Who needs 4GB of ram? If you need more than that shouldn't you be looking at an opteron system anyways.

 

[uOpt]

Uh, lots of uses. If you map enough data and actually touch it 4 GB are gone pretty quickly.

And I don't see how the 6 GB Opertons in our office are inferior to the 6 GB Xeons. (Apart from the fact that our particular brand of Xeons is trash).

Real big iron is not cost-effective and I don't want to mess with Solaris or AIX.

 

[Bernoulli]

Originally posted by: Viper96720
Who needs 4GB of ram? If you need more than that shouldn't you be looking at an opteron system anyways.

Not necessarily, but that's not the point.