Why I can only see "??" at any address before 0x70000 in Visual Studio Debug Memeory window?

I am debuging with Visual Studio 2010. I want to see the raw memory bytes in the Debug Memory window. But I noticed that before 0x70000, the memory content is not shown, only a "??" mark for each byt. Why can't I see the开发者_如何学JAVA content?

Visual Studio shows ?? for memory locations that are not mapped into your current address space. Since you can inspect any memory location within your process' 4 GB virtual address space, it only shows hex bytes for mapped memory.