書籍、作って理解するOS（ISBN:978-4-297-10847-2）に従ってOSを実装していく。
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例外のベクター： Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1: Basic Architecture, 6.5.1(P.165)
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特権命令： Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A: System Programming Guide, Part 1, 5.9(p.185)
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IOPL の変更： CPL = 0 のときに、
POPFもしくはIRETで変更できる -
IN/OUT 命令が一般保護例外を出すタイミング： If the CPL is greater than (has less privilege) the I/O privilege level (IOPL) and any of the corresponding I/O permission bits in TSS for the I/O port being accessed is 1.
- If in protected mode and the CPL is less than or equal to the current IOPL, the processor allows all I/O operations to proceed. If the CPL is greater than the IOPL or if the processor is operating in virtual-8086 mode, the processor checks the I/O permission bit map to determine if access to a particular I/O port is allowed. Each bit in the map corresponds to an I/O port byte address. For example, the control bit for I/O port address 29H in the I/O address space is found at bit position 1 of the sixth byte in the bit map. Before granting I/O access, the processor tests all the bits corresponding to the I/O port being addressed. For a doubleword access, for example, the processors tests the four bits corresponding to the four adjacent 8-bit port addresses. If any tested bit is set, a general-protection exception (#GP) is signaled. If all tested bits are clear, the I/O operation is allowed to proceed