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Windows64 and WoW
Over the past two years, Microsoft has released several 64 bit versions of Windows. Some of these versions have been for the IA64 platform only, others were similar to the one we are looking at today, starting with the 64bit version of Windows.net to accommodate the x86-64 ISA. To make a long story short, the operating system itself has been ported to 64 bit with full backward compatibility with the 32-bit and 16-bit legacy code. There are a few exceptions, of rarely used, highly specialized code that is no longer supported but it appears fair to say that way more than 99% of all legacy code will run in 64-bit Windows XP.
The AMD x86-64 architecture can work on 64-bit, 32-bit and 16-bit operating systems within each of which legacy modes are supported for backward compatibility.
In general, the 64-bit mode has been enabled by extending the instruction set by adding the so-called long mode to the number of execution modes. Long mode has two submodes, that is the 64 bit mode and the "compatibility mode" in which 32 and 16-bit operations are executed. In addition, the architecture supports legacy mode which encompasses running on a 32-bit or 16 bit operating system in either "Protected mode, "Virtual 8086" mode or "Real Mode" (for 16-bit legacy OS only). In other words, the CPU can run 32-bit and 16-bit operating systems as well as a 64-bit OS.
Windows64 and Windows on Windows interface
WindowsXP-64 handles 32-bit applications by means of the compatibility mode, that is a dynamic link library (DLL) called Windows on Windows or WoW that converts 32-bit function arguments to 64-bit format and the return values back to 32-bit data. Keep in mind that the extended registers are available to 64-bit mode programs only. In order to access the extended registers, a single override-byte called the REX (register extension) suffices, which allows the use of any of the extended GPR, MMX or debug registers.
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