- Physical Address Extension
In computing, Physical Address Extension (PAE) is a feature to allow (32-bit) x86 processors to access a physical address space (including random access memory and memory mapped devices) larger than 4 gigabytes.
First implemented in the Intel Pentium Pro in 1995, it was extended by AMD to add a level to the page table hierarchy, to allow it to handle up to 52-bit physical addresses, add NX bit functionality, and make it the mandatory memory paging model in long mode. PAE is provided by Intel Pentium Pro and above CPUs, including all later Pentium-series processors (except the 400 MHz-bus versions of the Pentium M). It is also available on other processors with similar or more advanced versions of the same architecture, such as the AMD Athlon[dubious ] and later AMD processor models.
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. 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 4GB even in PAE mode. According to Geoff Chappell, Microsoft limits 32-bit versions of Windows to 4GB 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 4GB. Unofficial kernel patches for Windows Vista and Windows 7 32-bit are available that break this enforced limitation, though the stability is not guaranteed.
For 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, while on Unix-like systems a variety of techniques are used, such as using mmap() to map regions of a file into and out of the address space as needed.
Page table structures
In traditional 32-bit protected mode, x86 processors use a two-level page translation scheme, where the control register
CR3points to a single 4 kB long page directory divided into 1024 × 4 byte entries that point to 4 kB long page tables, similarly consisting of 1024 × 4 byte entries pointing to 4 KB long pages.
Enabling PAE (by setting bit 5, PAE, of the system register
CR4) causes major changes to this scheme. By default, the size of each page remains as 4 kB. Each entry in the page table and page directory grows to 64 bits (8 bytes) rather than 32 bits – to allow for additional address bits. However, the size of tables does not change, so both table and directory now have only 512 entries. Because this allows only one quarter of the entries of the original scheme, an extra level of hierarchy has been added, so CR3 now points to the Page Directory Pointer Table, a short table which contains pointers to 4 page directories.
The entries in the page directory have an additional flag in bit 7, named
PS(for page size). If the system has set this bit to 1, the page directory entry does not point to a page table, but to a single large 2 MB page (Page Size Extension). The NX bit is another flag in the page directory, in bit 63, to mark pages as no execute. Because the 12 least significant bits of the page table entry's 64 bits are either similar flags or are available for OS-specific data, a maximum of 52 bits are available to address 252 bytes, or 4 petabytes, of physical memory.
Software can identify via the CPUID flag PAE whether a CPU supports PAE mode or not.
On x86-64 processors in native long mode, the address translation scheme uses PAE but adds a fourth table, the 512-entry page-map level 4 table, and extends the page directory pointer table to 512 entries instead of the original 4 entries it has in protected mode. Currently 48 bits of virtual page number are translated, giving a virtual address space of up to 256 TB. In the page table entries, in the original specification, 40 bits of physical page number are implemented.
FreeBSD supports PAE in the 4.x series starting with 4.9, in the 5.x series starting with 5.1, and in all 6.x and later releases. Support requires the kernel PAE configuration-option. Loadable kernel modules can only be loaded into a kernel with PAE enabled if the modules were built with PAE enabled; the binary modules in FreeBSD distributions are not built with PAE enabled, and thus cannot be loaded into PAE kernels. Not all drivers support more than 4 GB of physical memory; those drivers won't work correctly on a system with PAE.
Initial support for PAE was added to the Haiku operating system sometime after the R1 Alpha 2 release. With the release of R1 Alpha 3 PAE is now officially supported.
The Linux kernel includes full PAE mode support starting with version 2.3.23, enabling access of up to 64 GB of memory on 32-bit machines. A PAE-enabled Linux kernel requires that the CPU also support PAE. As of 2009[update], some common Linux distributions have started to use a PAE-enabled kernel as the distribution-specific default because it adds the NX bit. 
Mac OS X
Versions 10.4.4 through 10.5.8 of Mac OS X will run on both x86 and PowerPC processors. Version 10.6 and 10.7 version of OS X only run on an x86 processors. So far, all x86 Macs have used Intel (not AMD) CPUs. OS X versions that are compatible with x86 fully support PAE and the NX bit on all Intel Macs. Mac Pro and Xserve systems can use up to 64 GB of RAM. The Mac OS X 10.5 (Leopard) kernel remains 32-bit. Mac OS X 10.6 (Snow Leopard) can be booted into a 64-bit version of the kernel on certain systems; Mac OS X 10.7 (Lion) ships with a 64-bit enabled kernel by default.
The x86 versions of the following releases of Microsoft Windows support PAE: Itanium versions of these operating systems (if they exist) do not use PAE because the Itanium does not need nor implement PAE. x86-64 editions of Windows always implement PAE, because it is a mandatory feature of Long mode.
Windows Version  32-bit Editions 64-bit Editions Windows 2000 Professional, Server 4 GB N/A Windows 2000 Advanced Server 8 GB N/A Windows 2000 Datacenter 32 GB N/A Windows XP Starter 512 MB N/A Windows XP Home & Media Center 4 GB N/A Windows XP Professional 4 GB 128 GB Windows Server 2003 Web 2 GB N/A Windows Server 2003 Small Business, Home, Storage 4 GB N/A Windows Server 2003 Storage Server 4 GB N/A Windows Server 2003 R2 Standard Edition 4 GB 32 GB Windows Server 2003 Standard Edition (SP1) 4 GB 32 GB Windows Server 2003 Standard Edition (SP2) 4 GB 32 GB Windows Server 2003 Enterprise Edition (SP1) 16 GB with 4GT N/A Windows Server 2003 R2 Enterprise Edition 64 GB 1 TB Windows Server 2003 Datacenter (SP1) 128 gb N/A Windows Server 2003 R2 Datacenter (SP1) 128 GB 1 TB Windows Server 2003 R2 Datacenter (SP2) 128 GB 2 TB Windows Vista Starter 1 GB N/A Windows Vista Home Basic 4 GB 8 GB Windows Vista Home Premium 4 GB 16 GB Windows Vista Business, Enterprise, Ultimate 4 GB 128 GB Windows Server 2008 Standard, Web 4 GB 32 GB Windows Server 2008 Enterprise, Datacenter 64 GB 2 TB Windows 7 Starter 2 GB N/A Windows 7 Home Basic 4 GB 8 GB Windows 7 Home Premium 4 GB 16 GB Windows 7 Professional, Enterprise, Ultimate 4 GB 192 GB Windows Server 2008 R2 Foundation N/A 8 GB Windows Server 2008 R2 Standard N/A 32 GB Windows Server 2008 R2 Enterprise, Datacenter, or Itanium N/A 2 TB
Annotations: Values for Windows Server 2008 64-bit also apply to Windows Server 2008 R2 (which dropped 32-bit support). Values for Windows Server 2003 64-bit depend on:
- service pack level
- the release being R2 or not
The highest possible values appear here. The original releases of Windows XP and Windows XP SP1 used PAE mode to allow RAM to extend beyond the 4GB address limit. However, it led to compatibility problems with 3rd party drivers which led Microsoft to remove this capability in Windows XP Service Pack 2. Windows XP SP2 and later, by default, on processors with the no-execute (NX) or execute-disable (XD) feature, runs in PAE mode in order to allow NX. The no execute (NX, or XD for execution disable) bit resides in bit 63 of the page table entry and, without PAE, page table entries on 32-bit systems have only 32 bits; therefore PAE mode is required in order to exploit the NX feature. However, "client" versions of 32-bit Windows (Windows XP SP2 and later, Windows Vista, Windows 7) limit physical address space to the first 4 GB for driver compatibility  and licensing reasons, even though these versions do run in PAE mode if NX support is enabled.
Solaris supports PAE beginning with Solaris version 7. However, third-party drivers used with version 7 which do not specifically include PAE support may operate erratically or fail outright on a system with PAE.
- PSE-36: an alternative to Physical Address Extension
- Page Size Extension
- Architecture of Windows NT
- 3 GB barrier: why Windows often sees less than 4 GB RAM, even with PAE enabled
- ^ "AMD64 Architecture Programmer's Manual Volume 2: System Programming" (PDF). AMD64 Architecture Programmer's Manual. Advanced Micro Devices. November 1, 2009. pp. 124–143. http://support.amd.com/us/Processor_TechDocs/24593.pdf. Retrieved February 3, 2010. "Long-mode page translation requires the use of physical-address extensions (PAE). Before activating long mode, PAE must be enabled by setting CR4.PAE to 1. Activating long mode before enabling PAE causes a general-protection exception (#GP) to occur."
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