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来源：Intel® 64 and IA-32 Architectures Software Developer's Manual Volume 3A: System Programming Guide, Part 1

 

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Chapter 2.2 MODES OF OPERATION 运行模式

 

The IA-32 supports three operating modes and one quasi-operating mode（三个运行模式和一个准运行模式）:

• Protected mode（保护模式） — This is the native operating mode of the processor. It provides a rich set of architectural features, flexibility, high performance and backward compatibility to existing software base.
• Real-address mode（实地址模式） — This operating mode provides the programming environment of the Intel 8086 processor, with a few extensions (such as the ability to switch to protected or system management mode).
• System management mode (SMM)（系统管理模式） — SMM is a standard architectural feature in all IA-32 processors（SMM是所有IA-32处理器的标准体系结构特征）, beginning with the Intel386 SL processor. This mode provides an operating system or executive with a transparent mechanism for implementing power management and OEM differentiation features. SMM is entered through activation of an external system interrupt pin (SMI#), which generates a system management interrupt (SMI). In SMM, the processor switches to a separate address space while saving the context of the currently running program or task. SMM-specific code may then be executed transparently. Upon returning from SMM, the processor is placed back into its state prior to the SMI.
• Virtual-8086 mode（虚拟8086模式） — In protected mode, the processor supports a quasi-operating mode known as virtual-8086 mode（在保护模式下的虚拟8086模式）. This mode allows the processor execute 8086 software in a protected, multitasking environment.（在保护的多任务环境下执行8086程序）

Intel 64 architecture supports all operating modes of IA-32 architecture and IA-32e modes:

• IA-32e mode — In IA-32e mode, the processor supports two sub-modes: compatibility mode and 64-bit mode. 64-bit mode provides 64-bit linear addressing and support for physical address space larger than 64 GBytes. Compatibility mode allows most legacy protected-mode applications to run unchanged.

Figure 2-3 shows how the processor moves between operating modes.

 

 

Figure 2-3. Transitions Among the Processor’s Operating Modes

 

The processor is placed in real-address mode following power-up or a reset. The PE flag in control register CR0（CR0控制寄存器中的PE标志位） then controls whether the processor is operating in real-address or protected mode. See also: Section 9.9, “Mode Switching.”

The VM flag in the EFLAGS register（EFLAGS寄存器中的VM标志位） determines whether the processor is operating in protected mode or virtual-8086 mode. Transitions between protected mode and virtual-8086 mode are generally carried out as part of a task switch or a return from an interrupt or exception handler. See also: Section 15.2.5, “Entering Virtual-8086 Mode.”

The LMA bit (IA32_EFER.LMA.LMA[bit 10]) determines whether the processor is operating in IA-32e mode. When running in IA-32e mode, 64-bit or compatibility sub-mode operation is determined by CS.L bit of the code segment. The processor enters into IA-32e mode from protected mode by enabling paging and setting the LME bit (IA32_EFER.LME[bit 8]). See also: Chapter 9, “Processor Management and Initialization.”

The processor switches to SMM whenever it receives an SMI while the processor is in real-address, protected, virtual-8086, or IA-32e modes（无论在上述何种模式下，处理器收到SMI中断就切换到SMM模式）. Upon execution of the RSM instruction（RSM指令）, the processor always returns to the mode it was in when the SMI occurred.