HP dc5100 - Microtower PC White Paper page 2

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.
The current world consists of 32-bit and 64-bit operating systems (OSes). Both the home and professional
versions of Microsoft Windows XP are 32-bit operating systems. Microsoft Windows XP Professional x64
is a 64-bit OS. Windows XP Home/Professional, along with all other 32-bit OSes, supports a memory
address range of up to 4 GB for both virtual and physical memory addresses. In practice, available RAM
is always less than 4 GB because system components require some available memory addresses, result-
ing in less memory available for the operating system. The reasons for this limitation are explained below.
Windows XP Professional x64 is a 64-bit operating system and it supports up to 32 GB of memory
addressing. However, this is just one component to large addressing. In addition to the OS, the entire sys-
tem must support greater than 4 GB of memory addressing for large addressing to occur. 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 capa-
bility. 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. This proces-
sor can run 64-bit OSes.
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.
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