Frequently Asked Questions for FreeBSD 6.`X` and 7.`X`
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# 4.2 Memory

4.2.1. Does FreeBSD support more than 4 GB of memory (RAM)? More than 16 GB? More than 48 GB?
4.2.2. Why does FreeBSD report less than 4 GB memory when installed on an i386 machine?

4.2.1. Does FreeBSD support more than 4 GB of memory (RAM)? More than 16 GB? More than 48 GB?

Yes. FreeBSD as an operating system generally supports as much physical memory (RAM) as the platform it is running on does. Keep in mind that different platforms have different limits for memory; for example i386™ without PAE supports at most 4 GB of memory (and usually less than that because of PCI address space) and i386 with PAE supports at most 64 GB memory. AMD64 platforms currently deployed support up to 1 TB of physical memory.

4.2.2. Why does FreeBSD report less than 4 GB memory when installed on an i386 machine?

The total address space on i386 machines is 32-bit, meaning that at most 4 GB of memory is addressable (can be accessed). Furthermore, some addresses in this range are reserved by hardware for different purposes, for example for using and controlling PCI devices, for accessing video memory, and so on. Therefore, the total amount of memory usable by the operating system for its kernel and applications is limited to significantly less than 4 GB. Usually, 3.2 GB to 3.7 GB is the maximum usable physical memory in this configuration.

To access more than 3.2 GB to 3.7 GB of installed memory (meaning up to 4 GB but also more than 4 GB), a special tweak called PAE must be used. PAE stands for Physical Address Extension and is a way for 32-bit x86 CPUs to address more than 4 GB of memory. It remaps the memory that would otherwise be overlayed by address reservations for hardware devices above the 4 GB range and uses it as additional physical memory (see pae(4)). Using PAE has some drawbacks; this mode of memory access is a little bit slower than the normal (without PAE) mode and loadable modules (see kld(4)) are not supported. This means all drivers must be compiled into the kernel.

The most common way to enable PAE is to build a new kernel with the special ready-provided kernel configuration file called `PAE`, which is already configured to build a safe kernel. Note that some entries in this kernel configuration file are too conservative and some drivers marked as unready to be used with PAE are actually usable. A rule of thumb is that if the driver is usable on 64-bit architectures (like AMD64), it is also usable with PAE. If you wish to create your own kernel configuration file, you can enable PAE by adding the following line to your configuration:

    options       PAE

PAE is not much used nowadays because most new x86 hardware also supports running in 64-bit mode, known as AMD64 or Intel® 64. It has a much larger address space and does not need such tweaks. FreeBSD supports AMD64 and it is recommended that this version of FreeBSD be used instead of the i386 version if 4 GB or more memory is required.

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