3 GB barrier
In computing, the 3 GB barrier is a limitation of some 32-bit operating systems running on x86 microprocessors. It prevents the operating systems from using more than about 3 GB (3 × 10243 bytes) of main memory (RAM). The exact barrier varies by motherboard and I/O device configuration, particularly the size of video RAM; it may be in the range of 2.75 GB to 3.5 GB.
The barrier can be resolved by moving to a 64-bit processor and 64-bit operating system. On certain x86 hardware, it is possible to resolve it by using an operating system such as Linux or certain versions of Windows Server and Mac OS that fully support physical address extension (PAE) mode on x86.
The barrier is caused by a set of interactions between several components, including the operating system.
Physical address limits
It is a common misconception that 32-bit processors and operating systems are limited to 4 GB (232 bytes) of RAM, as were the original 80386DX and other early CPUs of that data word width, and that this is a primary cause of the "3 GB barrier". This is not a true limit of these processors - just as 16-bit CPUs were themselves not limited to 64 KB (216 bytes), 8-bits to 256 bytes or 4-bits to 16 bytes - but a mere artifact of historical convenience, as the expanded main data bus width allowed multiplexing of sufficiently wide memory addresses for what was widely considered (at the time of their introduction) to be a large address space, without the need for additional dedicated address lines. Compare the 80386SX, a 32 bit processor which re-used the 16-bit AT bus introduced with the 80286, including an additional 8 address-only lines, for a total of 24 address bits and an overall memory limit of 16 MB (224 bytes), and only one half of each 32-bit word transferred per bus cycle. Other 16-bit processors could commonly address at least 1 MB (220 bytes), 8-bits at least 64 KB, and even the original Intel 4004 had a maximum working RAM space of 1024 (210) "4+1"-bit words.
Similarly, since the 1995 Pentium Pro, almost all modern x86 processors can in fact already address up to 64 GB (236 bytes) RAM via physical address extension (PAE). PAE is a modification of the protected mode address translation scheme. It allows virtual or linear addresses to be translated to 36-bit physical addresses, instead of the 32-bit addresses available without PAE. The CPU pinouts likewise provide 36 bits of physical address to the motherboard. 
Many x86 operating systems, including any version of Linux with a PAE kernel and some versions of Windows Server and Mac OS, support the use of PAE to address up to 64 GB of RAM on an x86 system.
Use of PAE to address RAM above the 4 GB point allows use of more than 3 GB. There are, however, factors that limit this ability, and lead to the "3 GB barrier" under certain circumstances, even though the processor fully supports PAE.
Chipset and other motherboard issues
Although, as noted above, most x86 processors from the Pentium Pro onward do support physical addresses up to 64 GB, the rest of the motherboard must participate in allowing RAM above the 4GB point to be addressed by the CPU. Chipsets and motherboards allowing more than 4 GB of RAM with x86 processors do exist, but in the past, most of those intended for other than the high-end server market supported only 4 GB of RAM.
This, however, is not sufficient to explain the "3 GB barrier" that appears even when running some x86 versions of Microsoft Windows on platforms that do support more than 4 GB of RAM.
Memory mapped I/O and disabled RAM
Modern personal computers are built around a set of standards that depend on, among other things, the characteristics of the original PCI bus. The original PCI bus supported 32-bit physical addresses and 32-bit wide data transfers. PCI (and PCI Express, and AGP) devices present at least some, if not all, of their host control interfaces via a set of memory-mapped I/O locations (MMIO). The address space in which these MMIO locations appear is the same address space as that used by RAM, and while RAM can exist and be addressable above the 4 GB point, these MMIO locations decoded by I/O devices cannot be. They are limited by PCI bus specifications to addresses of 0xFFFFFFFF (232−1) and below. With 4 GB or more of RAM installed, and with RAM occupying a contiguous range of addresses starting at 0, some of the MMIO locations will overlap with RAM addresses. On machines with large amounts of video memory, MMIO locations have been found to occupy as much as 1.8 GB of the 32-bit address space.
The BIOS and chipset are responsible for detecting these address conflicts and disabling access to the RAM at those locations. Due to the way bus address ranges are determined on the PCI bus, this disabling is often at a relatively large granularity, resulting in relatively large amounts of RAM being disabled.
x86 chipsets that support more than 4 GB of RAM typically also support memory remapping (referred to in some BIOS setup screens as "memory hole remapping"). In this scheme, the BIOS detects the memory address conflict and in effect relocates the interfering RAM so that it may be addressed by the processor at a new physical address that does not conflict with MMIO. On the Intel side, this support once was limited to server chipsets; however, newer desktop chipsets like the Intel 955X and 965 and later support it as well. On the AMD side, the AMD K8 and later processors' built-in memory controller supported it from the beginning.
As the new physical addresses are above the 4 GB point, addressing this RAM does require that the operating system be able to use physical addresses larger than 232. This capability is provided by PAE. Note that there is not necessarily a requirement for the operating system to support more than 4 GB total of RAM, as the total RAM might be only 4 GB; it is just that a portion of it appears to the CPU at addresses in the range from 4 GB and up.
This form of the 3 GB barrier affects one generation of MacBooks, lasting 1 year (Core2Duo (Merom) – Nov 2006 to Oct 2007): the prior generation was limited to 2 GB, while later generations (Nov 2007–Oct 2009) allowed 4 GB by supporting PAE and memory hole remapping, and subsequent generations (late 2009 onwards) use 64-bit processors and support over 4 GB.
Windows version dependencies
In Microsoft's "non-server", or "client", x86 editions of Microsoft Windows: Windows XP, Windows Vista, Windows 7, Windows 8, and Windows 8.1, the 32-bit (x86) versions of these are able to operate x86 processors in PAE mode, and do so by default as long as the CPU present supports the NX bit. Nevertheless, these operating systems do not permit addressing of physical memory above the 4 GB address boundary. This is not an architectural limit; it is a limit imposed by Microsoft as a workaround for device driver compatibility issues that were discovered during testing.
Thus, the "3 GB barrier" under x86 Windows "client" operating systems can therefore arise in two slightly different scenarios. In both, RAM near the 4 GB point conflicts with memory-mapped I/O space. Either the BIOS simply disables the conflicting RAM; or, the BIOS remaps the conflicting RAM to physical addresses above the 4 GB point, but x86 Windows client editions refuse to use physical addresses higher than that, even though they are running with PAE enabled. The conflicting RAM is therefore unavailable to the operating system whether it is remapped or not.
- 640 KB barrier
- PSE-36 — an alternative to PAE on x86 processors to extend the physical memory addressing capabilities from 32 bits to 36 bits
- PCI hole
- Ring (computer security)
- RAM disk — a use for remapped RAM
- RAM limit
- Virtual memory — which governs the memory available to processes
- User space — and kernel space, which imposes another limit
- Matthew Murray (2009-10-27). "Windows 7: The 64-Bit Question". PCMag. Retrieved 2010-06-07. "A 32-bit system is limited to utilizing 4GB of RAM (232 addresses)"
- Andy Patrizio (2002-07-22). "AMD Answers the 64-Bit Question". Wired. Retrieved 2010-05-24. "32-bit processors like Intel's Pentium III/IV and AMD's Athlon have a memory limit of 4 GB per CPU. Any more memory can't be addressed."
- Shanley, Tom (1998). Pentium Pro and Pentium II System Architecture. PC System Architecture Series (Second ed.). Addison-Wesley. p. 445. ISBN 0-201-30973-4.
- "Volume 1: Specifications" (pdf). Pentium Pro Family Developer’s Manual. Intel Corporation. 1996. pp. 3–15. "The Pentium Pro processor physical address space is 236 bytes or 64-Gigabytes (64 Gbyte)."
- Microsoft Corporation (2008-08-22). "Memory Limits for Windows Releases". Retrieved 2010-06-07. "Windows Server 2008 Enterprise; Limit in 32-bit Windows: 64 GB"
- "Enabling PAE". Ubuntu Documentation. 2010-05-19. Retrieved 2010-06-07. "Physical Address Extension is a technology which allows 32 bit operating systems to use up to 64 Gb of memory (RAM)... PAE is supported on the majority of computers today and it is an easy procedure to enable it in Ubuntu, if it is not already."
- "Linux Kernel". Fedora Documentation. 2010-05-18. Retrieved 2010-06-07. "Fedora 8 includes the following kernel builds: ... The kernel-PAE, for use in 32-bit x86 systems with more than 4GB of RAM, or with CPUs that have an NX (No eXecute) feature."
- Intel Corporation (February 2005). "Intel Chipset 4 GB System Memory Support" (pdf). Pentium Pro Family Developer’s Manual. p. 7. "In uni-processor based systems for mobile, desktop, workstation, and entry level servers, chipsets may be limited to 4 GB of maximum memory. In today’s dual processor Intel server chipsets and workstations, maximum system memory size can be upwards of 16 GB."
- Mark Russinovich (2008-07-21). "Pushing the Limits of Windows: Physical Memory". Archived from the original on 9 June 2010. Retrieved 2010-06-07. "Windows XP SP2 also enabled Physical Address Extensions (PAE) support by default on hardware that implements no-execute memory because its required for Data Execution Prevention (DEP), but that also enables support for more than 4GB of memory."
- Intel Corporation (February 2005). "Intel Chipset 4 GB System Memory Support" (pdf). "In platforms populated with physical memory sizes approaching 4 GB and greater, onboard system resource requirements will likely not allow the operating system to take advantage of all physical memory populated due to PCI specification requirements and other memory mapped IO resources. Portions of physical memory may overlap with the memory space dedicated to other subsystems and become unavailable to the operating system."
- Intel Corporation (February 2005). "Intel Chipset 4 GB System Memory Support" (pdf). Pentium Pro Family Developer’s Manual. p. 8.
- Intel Corporation (February 2005). "Intel Chipset 4 GB System Memory Support" (pdf). Pentium Pro Family Developer’s Manual. p. 13,14. "In order to use remapping, the operating system must be able to address ranges higher than 4 GB of memory"
- "Understanding Intel Mac RAM".
- Mark Russinovich (2008-07-21). "Pushing the Limits of Windows: Physical Memory". Archived from the original on 9 June 2010. Retrieved 2010-06-07. "The problematic client driver ecosystem led to the decision for client SKUs to ignore physical memory that resides above 4GB, even though they can theoretically address it."