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11. Mixed-Mode Operation

  • Pentium allows mixed-mode operation in the same program.

  • Possible to combine 16-bit and 32-bit operands and addresses

  • D/B bit indicates the default size:

    • 0 indicates 16 bit mode,

    • 1 if 32-bit mode.

  • Pentium also provides two override instruction prefixes:

    • 66h changes default operands size from 16-bit to 32-bit and vice versa.

    • 67h changes 32-bit address to 16-bit and vice versa.

  • Segment descriptor format:

      Segment descriptor format

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