X86 Assembly Language - Execution Modes

Execution Modes

Further information: X86 architecture

The x86 processors support five modes of operation for x86 code, Real Mode, Protected Mode, Long Mode, Virtual 86 Mode, and System Management Mode, in which some instructions are available and others are not. A 16-bit subset of instructions are available in real mode (all x86 processors), 16-bit protected mode (80286 onwards), V86 mode (80386 and later) and SMM (Some Intel i386SL, i486 and later). In 32-bit protected mode (Intel 80386 onwards), 32-bit instructions (including later extensions) are also available; in long mode (AMD Opteron onwards), 64-bit instructions, and more registers, are also available. The instruction set is similar in each mode but memory addressing and word size vary, requiring different programming strategies.

The modes in which x86 code can be executed in are:

  • Real mode (16-bit)
  • Protected mode (16-bit and 32-bit)
  • Long mode (64-bit)
  • Virtual 8086 mode (16-bit)
  • System Management Mode (16-bit)

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