Athlon-64 memory question...

[Jeff7181]

I know the Athlon-64 only has a 40 bit memory address, not 64. And I know all the specs say because of it's 40 bit address, it's limited to 137 GB of RAM. But 2^40=1,099,511,627,776 which is 1 terrabyte. 2^37=137,438,953,472 which is 137 GB. So are the other 3 bits on the memory address bus for error correction or something?

 

[Lynx516]

Your error is one of basic arithmatic. 2^40 is 1Terrabit! not TerraByte. So you have to divde by 8 to get it into Terrabytes. And 8 is 2^3 so 2^40/2^3 = 2^37

 

[Jeff7181]

Originally posted by: Lynx516
Your error is one of basic arithmatic. 2^40 is 1Terrabit! not TerraByte. So you have to divde by 8 to get it into Terrabytes. And 8 is 2^3 so 2^40/2^3 = 2^37

Ahhh... ok, makes sense =)

 

[Peter]

And of course it's Tera not Terra.

Lynx, your arithmetics are flawed. With 2^40 addresses, you actually do get to 1 TeraBYTE. This is because one address refers to one byte, not one bit. It's been like that ever since the computers went from 4- to 8-bit datasets, sometime in the mid to late 1970s.

But either answer is incorrect.

The Athlon-64 as we have it now has 1 terabyte of address space. That doesn't mean it allows 1 terabyte of RAM.

Currently, with four DIMM slots per processor (remember, each processor brings another twin set of RAM controllers), and registered DIMM technology currently maxing out at 4 GBytes per stick, you'll get to 16 GBytes per CPU. Take Opteron 8xx CPUs for an 8-CPU system, and there's your 8x16=128 GBytes.

137? Flawed math once again folks. 137,438,953,472 bytes are 128 GBytes not 137. That's because a GByte is 2^30 bytes not 1^9 (unlike what the HDD marketing fellows want to shove down our throats).

 

[Howard]

GB = 10^9 bytes.
GiB = 2^30 bytes.

http://physics.nist.gov/cuu/Units/binary.html

 

[CTho9305]

Originally posted by: Howard
GB = 10^9 bytes.
GiB = 2^30 bytes.

http://physics.nist.gov/cuu/Units/binary.html

I've never seen anyone I would consider "normal" use those units 😉.

 

[Lynx516]

OK BRAIN FART. I thought it was a bit strange when righting it. Ignore what I said.