Virtual memory is not virtual address space (part 2). <\/P>\n
This myth is being perpetuated even as I write this series of articles<\/A>. <\/P>\n The user-mode virtual address space is normally 2GB, but that doesn’t limit you to 2GB of virtual memory. You can allocate memory without it being mapped into your virtual address space. (Those who grew up with Expanded Memory<\/A> or other forms of bank-switched memory are well-familiar with this technique.) <\/P> Provided you have enough physical memory and\/or swap file space, that 4GB memory allocation will succeed. <\/P>\n Of course, you can’t map it all into memory at once on a 32-bit machine, but you can do it in pieces. Let’s read a byte from this memory. <\/P> Of course, in a real program, you would have error checking and probably a caching layer in order to avoid spending all your time mapping and unmapping instead of actually doing work. <\/P>\nHANDLE h = CreateFileMapping(INVALID_HANDLE_VALUE, 0,\n PAGE_READWRITE, 1, 0, NULL);\n<\/PRE>\n
BYTE ReadByte(HANDLE h, DWORD offset)\n{\n SYSTEM_INFO si;\n GetSystemInfo(&si);\n DWORD chunkOffset = offset % si.dwAllocationGranularity;\n DWORD chunkStart = offset – chunkOffset;\n LPBYTE pb = (LPBYTE*)MapViewOfFile(h, FILE_MAP_READ, 0,\n chunkStart, chunkOffset + sizeof(BYTE));\n BYTE b = pb[chunkOffset];\n UnmapViewOfFile(pb);\n return b;\n}\n<\/PRE>\n