CODEWITHSUNDEEP
windowsmemorymemory-management
rubixibuc
rubixibuc

Reputation: 7397

Why is Windows XP (32-bit) limited to 3GB of usable memory?

It was written that with windows xp 32 bit the top 700 mb or ram or so was not usable. But it has also been said that there is no point installing more than 3gb or so. So this leads me to this issue. So is that area reserved by the os actually residing in that ram, or is just using the address space. In other words is the ram physically being filled with 700mb of information, or is 700mb of the cpu' address space being reserved and mapped. Because if it is not, then installing 3gbs would still be a problem, because it would still take away 700mb somehow?

Upvotes: 3

Views: 10003

Answers (2)

Chris Pitman
Chris Pitman

Reputation: 13104

32 bit systems have 4 GB of available address space, but address space is used fo rmore than just system memory. For example, part of video card memory can be mapped into the address space, which reduces the space available for system memory. This is covered in the section "How graphics cards and other devices affect memory limits" of Microsoft's Memory Limits for Windows page.

The applicable text is below to preserve for posterity:

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 don’t support physical memory above the 4GB mark, so they can’t access these remapped regions. Any X64 Windows or X86 Server release can.

Upvotes: 5

Calyth
Calyth

Reputation: 1673

The OS needs to reserve some physical address space for various functions, such as interrupt handlers, system calls, etc. Those address becomes unusable for programs on the OS.

Upvotes: 1

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