[Pc_Support] MB/CPU combo for general office use

[Bryan J. Smith]

Damien McKenna <dmckenna at thelimucompany.com> wrote:
> Unless I'm mistaken 

You are.

> you have to add DDR strips to an Athlon64-based system with
> DIMMs the same size, i.e. you can't mix a 512mb and 256mb,
> and they must be the same speed,

Synchronous clock, yes, of course.

But same size?  _Incorrect_.

You _only_ need to match DIMM size (including matching
IC/width on the DIMM) when you are using a Socket-939/940 and
the DIMMs are matched for dual-channel 128-bit access. 
That's because the CPU is reading from both DIMMs
simultaneously -- over exactly 368 traces (two 184-pin DDR
channels).

Now if you have 4 DIMM slots per CPU, you can have 2
completely different _pairs_ from each other.  E.g., 2x512
and 2x256.

On a Socket-754, you can do what you want, as it is only a
single DDR channel.  I.e., only 184 traces.

> but aside from that you should be able to use any of them.
> Note that I could be wrong on the size-mixing, I do
> remember it being an issue at least early on and I've not
> heard much on it since then.

It's not an "issue."  It's a requirement for Socket-939/940. 
The CPU is reading/writing a full 128-bit wide path.  So it
needs each pair of DIMMs, 64-bit each, to be _exactly_
matching -- IC size, width, latency, etc...

If you have 4 DIMMs, then you can have 2 different pairs of
DIMMs, each a matching set. 

> Correct, you've still got a darn fast CPU that can run
> 32bit code really well.  The difficulty comes with mixing
> 64bit software with 32bit software, specifically if you are
> running a 64bit OS and have to use a
> 32bit browser because of plugins (Flash comes to mind), and
> then its more of a case of it not being as fast as it
should
> be and maybe some path / symlink problems.

As I explained in my blog entry here:  

http://thebs413.blogspot.com/2005/10/what-is-x86-64-long-mode-memory-model.html



-- 
Bryan J. Smith     Professional, Technical Annoyance                      b.j.smith at ieee.org      http://thebs413.blogspot.com
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*** Speed doesn't kill, difference in speed does ***