[mikeg]

Armstrong, I'm not sure what you're referring to when you ask "Can Windows XP access the full 6GB of memory?" Is this in reference to something in the current thread? Windows XP 32-bit cannot use or even see any more than 4GB of RAM.

32 vs. 64 bit addressing is not a trivial topic to understand or explain. A little more detail might help.

First, as indicated in my last post, the 32-bit address space is limited to 4GB. This is true no matter what OS you're running. An address space larger than 4GB requires more than 32 bits to represent.

Lets look at a 32-bit Windows XP system with 4GB of RAM installed. In this case, the default configuration reserves 2GB for OS kernel, device drivers, etc. In fact, memory reserved for bootstrap loader, essential devices such as keyboard, etc. results in Windows having less than 2GB available for it's kernel and reporting somewhat less than the total 4GB of installed RAM in system properties.

This configuration will leave the remaining 2GB for applications. For years, applications have been written with code libraries designed to function within a maximum of 2GB application memory space. (Due to multitasking and many systems with less physical RAM, typically even less than 2GB is available in real-world conditions. But that gets into virtual memory and paging which which is a whole discussion unto itself.)

Other Factors: In an effort to keep up with increasing application memory demands, several techniques were introduced in Windows 32-bit operating systems as stop-gap measures until greater adoption of 64-bit addressing.

One solution was the Windows "3GB switch" which squeezes the OS kernel into 1 GB and reserves up to 3GB for memory-hungry apps. Relatively few apps are designed to detect and link to this additional memory. Using the 3 GB boot switch can also cause instability if used with device drivers that are expecting to use memory in that range.

Physical Address Extensions (PAE) is another stop-gap 32-bit technology that basically reserves some address space below the 4GB line to create address extensions above the 4GB line (think of these extensions as 4GB_address_space_2, 3, 4, etc.).

Generally speaking, only high-end 32-bit operating systems, such as Windows Server Enterprise can make use of PAE. Even then, relatively few apps are capable of even seeing this additional memory. Most user apps would still be confined to the original 2GB application memory space below the 4GB line.

Conclusion: The technology kludge briefly outlined above is why 64-bit is the way to go if you really need a system that can address more than 4GB of RAM. That's why I'll be using Win 7 64-bit on my new system (reasons for 6 to 8 GB of RAM were explained earlier).

Even though 64-bit is gradually emerging into the mainstream, most home Windows users should be fine with 2 to 4GB of RAM for quite some time to come. Meawhile, the number of PC's running comfortably with 1GB or less is shrinking, but if that's still enough for your needs, don't let anyone sell you more RAM or a new OS!