User manual HP COMPAQ DX6120 SLIM TOWER DESKTOP PC RAM ALLOCATION WITH MICROSOFT WINDOWS XP AND COMMERCIAL DESKTOPS

[. . . ] RAM Allocation with Microsoft Windows XP and HP Commercial Desktops Introduction . 3 1 Introduction This white paper discusses the limitation of RAM allocation using a 32-bit operating system such as Microsoft Windows XP on HP x86-based commercial desktop PCs. The Intel 945-based chipset, as well as older commercial desktop systems are covered by this paper. [. . . ] This includes the processor, chipset, physical memory capacity, and so on. HP commercial desktop systems with Intel 865, Intel 915, Intel 945, and ATI RS480 chipsets support up to 4 GB of RAM. There are currently no HP commercial desktop systems that support more than 4 GB of RAM. Certain systems can physically accommodate more than 4 GB of RAM on the system board, but the excess memory is not used because of 32-bit desktop chipset limitations. Why the 4-GB limit? In addition to the 64-bit OS requirement necessary to support greater than 4 GB system memory, there are other hardware requirements, including the processor, chipset, and the amount (greater than 4 GB) of physical system memory configured in the PC. All 32-bit x86 processors have 32-bits of addressing capability. With 32 address lines, 4 GB of memory space can be addressed logically: 232 = 4, 294, 967, 296 = 4 GB To go beyond the 4-GB limit, a processor must have greater than 32-bit addressing. There are two ways of accomplishing this: PAE and 64-bit processing. Physical Address Extension (PAE) is a mode allowing a 32-bit x86 processor to use an additional 4 bits for addressing, creating a 36-bit address: 236 = 68, 719, 476, 736 = 64 GB A 64-bit capable x86 processor, such an Intel Pentium 4 with EM64T, uses 64-bit addressing. 264 = 18, 446, 744, 073, 709, 551, 616 = 16 EB (exabyte 1018) Note: Although in theory a processor can address up to 16 EB, the OS and the chipset it is supporting can have a much lower limit. For example, Microsoft Windows Professional x64 can only address up to 32 GB, and a 32-bit chipset is still limited to 4 GB. Note: Do not confuse total physical memory and memory addressing with the amount of virtual memory available to the OS. No matter how much physical memory is in the system, a 32-bit OS will have 4 GB of virtual memory through paging. 2 How does the limit work? Physical memory addresses are divided into two sections: the PCI memory address range (also known as Memory Mapped I/O), and the RAM available to the operating system. This explains why even with 4 GB of memory, available RAM is less than 4 GB. PCI memory address space is used to transfer data by the BIOS, I/O cards, networking, PCI hubs, bus bridges, PCI Express, and graphics cards. [. . . ] Nothing herein should be construed as constituting an additional warranty. HP shall not be liable for technical or editorial errors or omissions contained herein. Microsoft, MS-DOS, Windows, and Windows NT are trademarks of Microsoft Corporation in the U. S. [. . . ]