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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 () 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 is not present with a 64-bit processor and 64-bit operating system, or with certain x86 hardware and 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 IA-32 CPUs. 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 lines 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. These are described in the following sections. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「3 GB barrier」の詳細全文を読む スポンサード リンク
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