Max Ram in 64bit ?

[Hardware_freak]

I have read in books that max amount of ram that a proccy can have is calculated by the formula 2^n (n - no of bits).

For eg: 2^32 gives 4GB which is right we can have max of 4GB ram in 32 bit.

but i have some doubts, when we cal 2^64 it is very high i.e 1.8 *10^10.
and on some sites even on vista site i have read that vista 64bit can have 128GB ram.

How they have calculated 128GB for 64 bit ?

 

[KingKrool]

1. 2^n is the maximum that can theoretically be supported in an n bit system.

2. There are tricks like PAE which allow you to go beyond that.

3. AMD and Intel do not actually allow a full 64 bits of address space, I believe the specification(s) currently allows only 48 bits.

4. An OS doesn't have to support the full range anyway.

 

[6pack]

practically there is no motherboard right now that can support that much ram

 

[Hardware_freak]

@KingKrool

Do u mean it is the drawback of processor & not O.S.

btw 2^48 is also very high.

128GB comes around 2^37 only.

 

[KingKrool]

As I said, an OS does not have to support the full range. They can safely do less. Supporting the entire range would mean more checks within the OS.

even in Linux, for example, in 32 bit systems, you can compile your kernel to be able to address just 1 GB RAM, 4 GB RAM or 64 GB RAM (PAE).