Artem Zezyulinskiy wrote:
> Le 12/05/2010 10:12, Michael Watterson a =E9crit :
>   =

>> Even fully 64bit OS and and CPU many laptops can only address 3Gbyte
>> physical RAM of the 4G RAM installed.
>>     =

>
> I don't understand what you saying.
>
> 64bit OS + 64bit CPU + 64bit Motherboard (data bus) are not able to =

> address 4G of RAM?
>
> Why?
>
>   =

Because many laptops are built down to a price and the gpu memory,  =

memory mapped I/o etc etc mapped to the 4th G of address space and there =

is no addressing for more than 4G RAM on mobo.  Some let you get 3.5Gbyte.

If you NEED more than 4G RAM make sure it's a "proper" mobo in PC =

/Laptop that allows more than 4G RAM, that allows x64 addressing rather =

than only PAE and doesn't have i/o & GPU  hiding 500M to 1G of your =

memory map. x64 chips do support the 4bit PAE  32->36bit. I dunno. Half =

baked designs adapted from 32linear/36bit PAE designs. I have no =

rational explanation for the  stupidity  of some  product's  designs.

NO x64 CPU does 64 bit addressing though. Most are limited to 256 T =

bytes virtual or physical (2 ^48 bits) with possible expansion on later =

designs to 2 ^ 51 bits.

Why do 32bit CPUs only seem to do 31bit addressing without the PAE? 2G =

is 31bits. 32bits is 4G.
http://en.wikipedia.org/wiki/Physical_Address_Extension

"x86 processor hardware-architecture is augmented with additional =

address lines used to select the additional memory, so physical address =

size increases from 32 bits to 36 bits. This, theoretically, increases =

maximum physical memory size from 4 GB to 64 GB. The 32-bit size of the =

virtual address is not changed, so regular application software =

continues to use instructions with 32-bit addresses and (in a flat =

memory model) is limited to 4 gigabytes of virtual address space. The =

operating system uses page tables to map this 4-GB address space into =

the 64 GB of physical memory. The mapping is typically applied =

differently for each process. In this way, the extra memory is useful =

even though no single regular application can access it all simultaneously."


In theory even a MOBO adapted from 64Gbyte 32bit PAE design can utilise =

64Gbyte RAM linearly with 64bit CPU (x64) and associated chipsets.  So =

consumer laptops that can only address 3G to 3.5G in 32bit mode and =

still only that in 64bits are shoddy. IMO even 32bit should give 4G RAM =

and PAE used to map GPU address space.

As an aside I had a pair of 200MHz Pentium Pro Servers and no-way could =

you have afforded upgrading from the 64M to the 4G never mind the =

theoretical 64G :-(



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