Intel IA-32 - Manual (Page 390)

9-16 Vol. 3A
PROCESSOR MANAGEMENT AND INITIALIZATION
Compatibility mode execution is selected on a code-segment basis. This mode allows legacy
applications to coexist with 64-bit applications running in 64-bit mode. An operating system
running in IA-32e mode can execute existing 16-bit and 32-bit applications by clearing their
code-segment descriptor’s CS.L bit to 0.
In compatibility mode, the following system-level mechanisms continue to operate using the
IA-32e-mode architectural semantics:
•
Linear-to-physical address translation uses the 64-bit mode extended page-translation
mechanism.
•
Interrupts and exceptions are handled using the 64-bit mode mechanisms.
•
System calls (calls through call gates and SYSENTER/SYSEXIT) are handled using the
IA-32e mode mechanisms.
9.8.5.4 Switching Out of IA-32e Mode Operation
To return from IA-32e mode to paged-protected mode operation. Operating systems must use
the following sequence:
1. Switch to compatibility mode.
2. Deactivate IA-32e mode by clearing CR0.PG = 0. This causes the processor to set
IA32_EFER.LMA = 0. The MOV CR0 instruction used to disable paging and subsequent
instructions must be located in an identity-mapped page.
3. Load CR3 with the physical base address of the legacy page-table-directory base address.
4. Disable IA-32e mode by setting IA32_EFER.LME = 0.
5. Enable legacy paged-protected mode by setting CR0.PG = 1
6. A branch instruction must follow the MOV CR0 that enables paging. Both the MOV CR0
and the branch instruction must be located in an identity-mapped page.