Design
x86 processor hardware-architecture is augmented with additional address lines used to select the additional memory, so physical address size increases from 32 bits to 36 bits. This, theoretically, increases maximum physical memory size from 4 GB to 64 GB - although the actual amount increased depends on the OS. The 32-bit size of the virtual address is not changed, so regular application software continues to use instructions with 32-bit addresses and (in a flat memory model) is limited to 4 gigabytes of virtual address space. The operating system uses page tables to map this 4-GB address space into the 64 GB of physical memory. The mapping is typically applied differently for each process. In this way, the extra memory is useful even though no single regular application can access it all simultaneously.
To use PAE, operating system support is required. Intel versions of Mac OS X support PAE. The Linux kernel supports PAE as a build option and most major distributions provide a PAE kernel either as the default or as an option (Red Hat Enterprise Linux 6+ kernels expect PAE). FreeBSD and NetBSD also support PAE as a kernel build option.
Microsoft Windows implements PAE if booted with the appropriate option, but current 32-bit desktop editions enforce the physical address space within 4 GB even in PAE mode. According to Geoff Chappell, Microsoft limits 32-bit versions of Windows to 4 GB as a matter of its licensing policy, and Microsoft Technical Fellow Mark Russinovich says that some drivers were found to be unstable when encountering physical addresses above 4 GB. Unofficial kernel patches for Windows Vista and Windows 7 32-bit are available that break this Microsoft enforced limitation, though the stability is not guaranteed. These tools increase the RAM limit of the 32-bit version of Windows 7 to 64 GB.
For 32-bit application software which needs access to more than 4 GB of RAM, operating systems may provide some special mechanisms in addition to the regular PAE support. On Windows this mechanism is called Address Windowing Extensions.
Read more about this topic: Physical Address Extension
Famous quotes containing the word design:
“Joe ... you remember I said you wouldn’t be cheated?... Nobody is really. Eventually all things work out. There’s a design in everything.”
—Sidney Buchman (1902–1975)
“I begin with a design for a hearse.
For Christ’s sake not black—
nor white either—and not polished!
Let it be weathered—like a farm wagon—”
—William Carlos Williams (1883–1963)