Hi,

If we run 32 bit system, can't we have more than 4 GB RAM? I think 32 bit can't support more than 4 GB in address. Please advice.

Yes the 32-bit kernels can support more than 4-GB of memory. For example RHEL4/5 has kernels built just for this very reason, the kernel-PAE (RHEL5) and kernel-hugemem (RHEL4) for example. Other distributions also offer support for greater then 4-GB of memory also. If I remember correctly the 32-bit kernel memory limit is 64-GB.

32-bits can only directly address 4GB (i.e. 2^32 = 4,294,967,296 bytes). Beyond that, a 32-bit system uses things like paging to address more memory.

A 64-bit system can directly address 1.845 X 10^19 bytes (16 exabytes) Motherboards are not expected to support this much memory for several years....

See:

http://kerneltrap.org/node/2450

http://groups.google.com/group/comp....625dfcec0b51eb

If memory serves, 32-bit "high memory" support under Linux will, at the cost of some slight performance, allow the kernel to access 64Gb of RAM. But each individual process still gets only a 4Gb address space, so you will need at least 32 memory-hungry programs to make proper use of this for anything more than I/O caching.

Also, it should be noted that some distributions, including Red Hat Enterprise Linux I think, disable high memory support by default, so you may need to recompile your kernel to enable it. Not difficult, but it can be daunting if you've not done it before (and it needs to be done by hand at each kernel update).

A kernel without high memory support will still work on a box with more than 4Gb of memory; it just won't use the rest.

Hope that helps,

—Robert J Lee

Moved: This thread is more suitable in <GENERAL> since it's not really Linux-specific, and has been moved accordingly to help your thread/question get the exposure it deserves.