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Vol. 3 9-17
PROCESSOR MANAGEMENT AND INITIALIZATION
from 64-bit mode through compatibility mode to legacy or real mode and then back
through compatibility mode to 64-bit mode.
9.9 MODE SWITCHING
To use the processor in protected mode after hardware or software reset, a mode
switch must be performed from real-address mode. Once in protected mode, soft-
ware generally does not need to return to real-address mode. To run software written
to run in real-address mode (8086 mode), it is generally more convenient to run the
software in virtual-8086 mode, than to switch back to real-address mode.
9.9.1 Switching to Protected Mode
Before switching to protected mode from real mode, a minimum set of system data
structures and code modules must be loaded into memory, as described in
Section
9.8, “Software Initialization for Protected-Mode Operation.” Once these tables are
created, software initialization code can switch into protected mode.
Protected mode is entered by executing a MOV CR0 instruction that sets the PE flag
in the CR0 register. (In the same instruction, the PG flag in register CR0 can be set to
enable paging.) Execution in protected mode begins with a CPL of 0.
Intel 64 and IA-32 processors have slightly different requirements for switching to
protected mode. To insure upwards and downwards code compatibility with Intel 64
and IA-32 processors, we recommend that you follow these steps:
1. Disable interrupts. A CLI instruction disables maskable hardware interrupts. NMI
interrupts can be disabled with external circuitry. (Software must guarantee that
no exceptions or interrupts are generated during the mode switching operation.)
2. Execute the LGDT instruction to load the GDTR register with the base address of
the GDT.
3. Execute a MOV CR0 instruction that sets the PE flag (and optionally the PG flag)
in control register CR0.
4. Immediately following the MOV CR0 instruction, execute a far JMP or far CALL
instruction. (This operation is typically a far jump or call to the next instruction in
the instruction stream.)
5. The JMP or CALL instruction immediately after the MOV CR0 instruction changes
the flow of execution and serializes the processor.
6. If paging is enabled, the code for the MOV CR0 instruction and the JMP or CALL
instruction must come from a page that is identity mapped (that is, the linear
address before the jump is the same as the physical address after paging and
protected mode is enabled). The target instruction for the JMP or CALL instruction
does not need to be identity mapped.