Reserved Instructions

Certain processor instructions are reserved for execution only by the operating system, and may therefore execute only at privilege level zero. Such instructions include HLT (Halt Processor), LGDT (Load GDT), and LTR (Load Task Register).

In addition, some I/O instructions are restricted:

  • The IOPL field in the EFLAGS register defines whether or not the current task has the right to use I/O-related instructions

  • The I/O Permission Bit Map in the TSS determines whether the current task may use ports in the I/O address space.

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