Many times query arise that how much RAM is supported by Redhat Enterprise Linux 5. Redhat knowledge base says follwing on this matter.
This depends on the capabilities of the CPU, the kernel running on the CPU, and how the application was compiled. CPUs such as the Intel Pentium 4 and the AMD Athlon are 32-bit processors, will use 32-bit kernels, and will run applications that are compiled and linked for a 32-bit environment. In contrast, most later processor models are capable of running 64-bit code This is often indicated as "AMD64", "EM64T", "x86-64" or even "x64". They can boot either 32-bit or 64-bit kernels, and, when using a 64-bit kernel, can execute both 32-bit and 64-bit applications.
In each of these cases, the virtual address space available to the executing application is different, as shown in the table below:
CPU Kernel Application Virtual Address Size
32 or 64 bit 32 bit (smp *) 32 bit slightly under 3GB
32 or 64 bit 32 bit (hugemem **) 32 bit slightly over 3.7GB
This depends on the capabilities of the CPU, the kernel running on the CPU, and how the application was compiled. CPUs such as the Intel Pentium 4 and the AMD Athlon are 32-bit processors, will use 32-bit kernels, and will run applications that are compiled and linked for a 32-bit environment. In contrast, most later processor models are capable of running 64-bit code This is often indicated as "AMD64", "EM64T", "x86-64" or even "x64". They can boot either 32-bit or 64-bit kernels, and, when using a 64-bit kernel, can execute both 32-bit and 64-bit applications.
In each of these cases, the virtual address space available to the executing application is different, as shown in the table below:
32 or 64 bit 32 bit (smp *) 32 bit slightly under 3GB
32 or 64 bit 32 bit (hugemem **) 32 bit slightly over 3.7GB
64 bit 64 bit 32 bit 4GB
64 bit 64 bit 64 bit more than 256GB
