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Modes of operation
Covered on Intel Volume 3. Specially useful is the "Transitions Among the Processor’s Operating Modes" diagram.
- Protected
- Real address
- System management
- IA-32e. Has two sub modes:
- Compatibility
- 64-bit
Real mode
CPU starts here for backwards compatibility.
IA-32e
Wikipedia seems to call it long mode: https://en.wikipedia.org/wiki/Long_mode
Contains two sub-modes: 64-bit and compatibility.
64-bit is the major mode of operation, compatibility mode emulates IA-32. This is where systems run most of the time.
The other mode is legacy mode, which as the name implies, should not be used on new programs.
Compatibility mode
Controlled by the CS.L bit of the segment descriptor.
It appears that it is possible for user programs to modify that during execution: http://stackoverflow.com/questions/12716419/can-you-enter-x64-32-bit-long-compatibility-sub-mode-outside-of-kernel-mode