The Mac OS X kernel (up to Snow Leopard) has used this feature to run 64-bit applications and (mainly) a 32-bit kernel. In Compatibility Mode, it is easy (and fast enough) to switch to the full 64-bit mode and back. In this mode, 32-bit applications can still run unmodified (discussed shortly), in a mode referred to as Compatibility Mode. Long Mode This is the native 64-bit mode of operation. Legacy Mode The CPU behaves like a 32-bit CPU and all the 64-bit enhancements are turned off. AMD64 is completely backward-compatible, allowing users to run 32-bit applications and operating systems unmodified, and has two main modes of operation: Intel developed the brand-new IA64 RISC architecture AMD took the x86 32-bit architecture, put it on 64-bit steroids (64-bit registers and integer operations, a 64-bit address space, etc.), and called it AMD64. As applications began to demand larger address spaces and RAM prices began to drop, Intel and AMD started to pursue 64-bit architectures.
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