64-bit Linux OS and 32-bit application

[bvramesh]

Hi,

I am new to this forum and need help from the Linux gurus over here.
If we have 64-bit SUSE Linux OS and 32-bit application running on a board which has 30 GB RAM, it is for sure that the OS can access the whole 30 GB RAM. But, the question is can the 32-bit application access the whole 30 GB RAM or only 4 GB of RAM? I guess only 4 GB. Could any one of you have a quick answer and the theory behind the answer?

Thanks & Regards,
Ramesh.

[gogalthorp]

32 bits can only address 4gig. It's the math 2^32

[bvramesh]

Thanks gogalthorp. But, If only ONE 32-bit application is running on a 64-bit SUSE OS on a board having 30 GB RAM, application can use 4 GB RAM and 26 GB RAM will be a waste and not used at all ???

[gogalthorp]

Yep, But the the OS will use it for disk cache and stuff so it can speed things up

The OS and 64bit apps can address the full space. Also just because an app can address a give address space does not mean it uses it all. It totally depends on the amount of data. Also there are tricks that can be used to page memory but that tends to slow things down. There is a version of the 32bit kernel that does that (pae) it allows the OS to go to 64gig but AFAIK the max any given process can use is 4G

[linuxforever]

Remember that a 64-bits OS is intended for use with a 64-bits CPU.
And using it this way makes the OS much faster.
Of course you can use 32-bits applications, but it is not recommended.
You can see it on the 64-bits suse DVD.
There are also 32-bits program versions but in a 64-bits installation file.
So they will be emulated in some way.

On the other hand windows xp is 32-bits and cannot access all installed memory.
I came along a motherboard which could have a max of 8G memory.
But xp could access only a max of 3G.
This sucks of course!

Windows came along with a 64-bits version of xp, but this was a complete disaster!
Not stable, no support, no drivers, etc.
And then came 64-bits vista, but this proved to be a disaster for other reasons.

On the other hand suse is releasing 64-bits versions for a long time.
And they run fine.

[gogalthorp]

In Linux also the max memory usable in a normal 32bit OS is 3gig 1 gig is reserved for the OS. So the largest process you can run in a 32 bit is OS with 4Gig memory is 3 gig. There are patches to both 32bit Windows and Linux to address more via the pae extensions of the Intel architecture. But these pae patches do slow things down since it is a bank switch scheme.