188:
of 0xFFFFFFFF (2 − 1) and below. With 4 GiB 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 other disabled RAM might include
248:, lasting 1 year (Core2Duo (Merom) – November 2006 to October 2007): the prior generation was limited to 2 GiB, while later generations (November 2007 – October 2009) allowed 4 GiB through the use of PAE and memory-hole remapping, and subsequent generations (late 2009 onwards) use 64-bit processors and therefore can address over 4 GiB.
187:
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 GiB point, these MMIO locations decoded by I/O devices cannot be. They are limited by PCI bus specifications to addresses
232:
As the new physical addresses are above the 4 GiB point, addressing this RAM does require that the operating system be able to use physical addresses larger than 2. This capability is provided by PAE. Note that there is not necessarily a requirement for the operating system to support more than
220:
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 feature once was limited to server chipsets; however,
281:
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 GiB point conflicts with memory-mapped I/O space. Either the BIOS simply disables the conflicting RAM; or, the BIOS remaps the conflicting
764:
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
369:
Devices have to map their memory below 4 GB for compatibility with non-PAE-aware
Windows releases. Therefore, if the system has 4GB of RAM, some of it is either disabled or is remapped above 4GB by the BIOS. If the memory is remapped, X64 Windows can use this memory. X86 client versions of Windows
900:
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. The problematic client driver ecosystem led to the
207:
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
282:
RAM to physical addresses above the 4 GiB 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.
107:
have 32 or fewer physical address bits; however, most x86 processors since the
Pentium Pro, which was first sold in 1995, have the Physical Address Extension (PAE) mechanism, which allows addressing up to 64 GiB (2 words) of memory. PAE is a modification of the
695:
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
86:
Whatever the actual position of the "barrier", there is no code in operating system software nor any hardware architectural limit that directly imposes it. Rather, the "barrier" is the result of interactions between several aspects of both.
728:
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
627:
Physical
Address Extension is a technology which allows 32 bit operating systems to use up to 64 GB of memory... PAE is supported on the majority of computers today and it is an easy procedure to enable it in Ubuntu, if it is not
138:
There are other factors that may limit this ability to use up to 64 GiB of memory, and lead to the "3 GB barrier" under certain circumstances, even on processors that implement PAE. These are described in the following sections.
159:
and motherboards allowing more than 4 GiB of RAM with x86 processors do exist, but in the past, most of those intended for other than the high-end server market could access only 4 GiB of RAM.
270:. Nevertheless, these operating systems do not permit addressing of physical memory above the 4 GiB address boundary. This is not an architectural limit; it is a limit imposed by Microsoft as a
163:
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 can access more than 4 GiB of RAM.
884:
712:
386:
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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.
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decision for client SKUs to ignore physical memory that resides above 4GB, even though they can theoretically address it. 4GB is the licensed limit for 32-bit client SKUs.
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x86 chipsets that can address more than 4 GiB of RAM typically also allow memory remapping (referred to in some BIOS setup screens as "
448:
468:
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.
155:
up to 64 GiB, the rest of the motherboard must participate in allowing RAM above the 4 GiB point to be addressed by the CPU.
171:
Modern personal computers are built around a set of standards that depend on, among other things, the characteristics of the original
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501:
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physical addresses, instead of the 32-bit addresses available without PAE. The CPU pinouts likewise provide 36 bits of physical
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physical memory may overlap with the memory space dedicated to other subsystems and become unavailable to the operating system.
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778:
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309:— an alternative to PAE on x86 processors to extend the physical memory addressing capabilities from 32 bits to 36 bits
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In order to use remapping, the operating system must be able to address ranges higher than 4 GB of memory.
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of RAM, as the total RAM might be only 4 GiB; it is just that a portion of it appears to the CPU at
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175:. The original PCI bus implemented 32-bit physical addresses and 32-bit-wide data transfers. PCI (and
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don't support physical memory above the 4GB mark, so they can't access these remapped regions.
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and later operate x86 processors in PAE mode by default when the CPU present implements the
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183:) devices present at least some, if not all, of their host control interfaces via a set of
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Many 32-bit computers have 32 physical address bits and are thus limited to 4 GiB (2
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address translation scheme which allows virtual or linear addresses to be translated to
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The
Pentium Pro processor physical address space is 2 bytes or 64-Gigabytes (64 Gbyte).
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423:
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128:
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40:
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newer desktop chipsets like the Intel 955X and 965 and later have it as well. On the
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59:; it may be in the range of 2.75 GB to 3.5 GB. The barrier is not present with a
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and later processors' built-in memory controller had it from the beginning.
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22:
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How to use full 4 GB RAM in
Windows 7 32 Bit (Gavotte RAMDisk in Windows 7)
16:
Limitation of some 32-bit operating systems running on x86 microprocessors
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44:
245:
156:
135:, can use PAE to address up to 64 GiB of memory on an x86 system.
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Why you should forget about 4 GiB of RAM on 32-bit systems and move on
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Many x86 operating systems, including any version of Linux with a PAE
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A 32-bit system is limited to utilizing 4GB of RAM (2 addresses)
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67:, or with certain x86 hardware and an operating system such as
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Windows Server 2008 Enterprise; Limit in 32-bit
Windows: 64 GB
37:
244:
This form of the 3 GB barrier affects one generation of
43:. It prevents the operating systems from using all of 4
811:
809:
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compatibility issues that were discovered during testing.
258:
Physical Address Extension § Operating system support
83:(PAE) mode on x86 to access more than 4 GiB of RAM.
147:
Although, as noted above, most x86 processors from the
55:
and I/O device configuration, particularly the size of
488:. PC System Architecture Series (Second ed.).
481:
331:— which governs the memory available to processes
885:"Pushing the Limits of Windows: Physical Memory"
713:"Pushing the Limits of Windows: Physical Memory"
387:"Pushing the Limits of Windows: Physical Memory"
337:— and kernel space, which imposes another limit
484:Pentium Pro and Pentium II System Architecture
51:) of main memory. The exact barrier varies by
8:
262:The "non-server", or "client", x86 SKUs of
818:"Intel Chipset 4 GB System Memory Support"
779:"Intel Chipset 4 GB System Memory Support"
743:"Intel Chipset 4 GB System Memory Support"
670:"Intel Chipset 4 GB System Memory Support"
349:
825:Pentium Pro Family Developer's Manual
786:Pentium Pro Family Developer's Manual
677:Pentium Pro Family Developer's Manual
560:Pentium Pro Family Developer's Manual
525:Pentium Pro Family Developer's Manual
241:in the range from 4 GiB and up.
7:
589:"Memory Limits for Windows Releases"
359:"Memory Limits for Windows Releases"
143:Chipset and other motherboard issues
816:Intel Corporation (February 2005).
777:Intel Corporation (February 2005).
741:Intel Corporation (February 2005).
668:Intel Corporation (February 2005).
167:Memory-mapped I/O and disabled RAM
14:
449:"AMD Answers the 64-Bit Question"
827:. p. 13, 14. Archived from
424:"Windows 7: The 64-Bit Question"
29:refers to a limitation of some
422:Murray, Matthew (2009-10-27).
103:. x86 processors prior to the
1:
856:"Understanding Intel Mac RAM"
447:Patrizio, Andy (2002-07-22).
575:Pin #: C1; Signal Name: A35#
566:. January 1996. p. 15–5
531:. January 1996. p. 3–15
252:Windows version dependencies
151:onward are able to generate
788:. p. 8. Archived from
679:. p. 7. Archived from
963:
553:"Volume 1: Specifications"
518:"Volume 1: Specifications"
255:
81:Physical Address Extension
881:Russinovich, Mark Eugene
709:Russinovich, Mark Eugene
325:— a use for remapped RAM
587:Microsoft Corporation.
357:Microsoft Corporation.
91:Physical address limits
71:or certain versions of
65:64-bit operating system
937:X86 memory management
480:Shanley, Tom (1998).
127:and some versions of
645:Fedora Documentation
616:Ubuntu Documentation
120:to the motherboard.
153:physical addresses
79:that allow use of
564:Intel Corporation
529:Intel Corporation
392:Microsoft TechNet
383:Russinovich, Mark
212:Address remapping
185:memory-mapped I/O
34:operating systems
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947:X86 architecture
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887:. Archived from
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858:. Archived from
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403:on 8 August 2019
399:. Archived from
385:(21 July 2008).
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61:64-bit processor
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41:microprocessors
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911:External links
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883:(2008-07-21).
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711:(2008-07-21).
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641:"Linux Kernel"
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612:"Enabling PAE"
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490:Addison-Wesley
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256:Main article:
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129:Windows Server
110:protected mode
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49:4 × 1024 bytes
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893:. Retrieved
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649:. Retrieved
647:. 2010-05-18
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27:3 GB barrier
26:
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18:
407:14 December
233:4 GiB
218:memory hole
177:PCI Express
149:Pentium Pro
105:Pentium Pro
53:motherboard
36:running on
25:, the term
931:Categories
895:2017-08-07
866:2010-03-11
838:2017-08-07
799:2017-08-07
759:2017-08-07
723:2017-08-07
690:2017-08-07
651:2010-06-07
622:2010-06-07
594:2017-08-07
570:2018-12-12
535:2018-12-12
492:. p.
463:2017-08-07
430:2017-08-07
364:2017-08-07
344:References
335:User space
272:workaround
264:Windows XP
225:side, the
208:disabled.
397:Microsoft
323:RAM drive
292:RAM limit
239:addresses
57:video RAM
23:computing
628:already.
313:PCI hole
286:See also
246:MacBooks
157:Chipsets
729:memory.
426:. PCMag
173:PCI bus
500:
307:PSE-36
302:x86-64
268:NX bit
227:AMD K8
125:kernel
114:36-bit
101:memory
31:32-bit
832:(PDF)
821:(PDF)
793:(PDF)
782:(PDF)
753:(PDF)
746:(PDF)
684:(PDF)
673:(PDF)
556:(PDF)
521:(PDF)
453:Wired
235:total
194:SMRAM
133:macOS
99:) of
97:words
77:macOS
69:Linux
498:ISBN
409:2022
274:for
205:BIOS
203:The
198:iGPU
196:and
190:APIC
179:and
131:and
75:and
63:and
696:GB.
494:445
223:AMD
181:AGP
45:GiB
38:x86
21:In
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808:^
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47:(
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