(Nitpickers: actually a little more than a megabyte.) This mapping behavior is required by the x86 processor’s virtual-8086 mode. (Windows 95 had to work around a lot of CPU bugs and firmware bugs.) Furthermore, the entire first megabyte of virtual address space is mapped to the logical address space of the active virtual machine. The Windows 95 virtual machine manager permanently maps the first 64 KB of physical memory to the first 64KB of virtual memory in order to avoid a CPU erratum. Therefore, on Windows NT, you could base your executable at 0x00010000, and many applications did just that.īut on Windows 95, there was a lot of stuff already there. The only thing that was already there was a PAGE_ NOACCESS page mapped at zero in order to catch null pointer accesses. Windows NT didn’t have a lot of stuff at low addresses. The first part means that you also want to avoid the things that are already there. Since the operating system itself puts DLLs at high addresses and the default base address for non-operating system DLLs is 0x10000000, this means that the base address for the executable should be somewhere below 0x10000000, and the lower you go, the more room you have before you start colliding with DLLs. For an executable, the not colliding with things that may arrive later part means avoiding the region of the address space that tends to fill with DLLs.
This means not colliding with things already in the address space (which will force you to relocate) as well as not colliding with things that may arrive in the address space later (forcing them to relocate). The goal in choosing a base address is to minimize the likelihood that modules will have to be relocated. But some choices for base address are better than others. The only technical requirement for the base address of an EXE is that it be a multiple of 64KB.
It has to do with the amount of address space mapped by a single page directory entry on an x86 and a design decision made in 1987. Why that particular value for EXEs? What’s so special about 4 megabytes The default base address for a DLL is 0x10000000, but the default base address for an EXE is 0x00400000.
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