The Knowledge About Protected Mode Must Be Master During Learning The Operating System

In-depth knowledge of the Intel CPU is base on the knowledge of protected mode.And it will only be able for further study when the knowledge of protected mode is ready.

Master the GDT(Global Descriptor Table),LDT( Local Descriptor Table) and the IDT(Interrupt Descriptor Table). Their limits and properties are helpful for descriptor which provide the function of limit and Protection. Realize the protection mechanism of memory paging mechanism,and the function of R/W and U/S in the PDE and PTE. Realize the protective effect of the linear address in the paged memory management for the physical address. The use of interrupt operation under the protected mode must do something with Privileges inspection first. It can not be unceremonious to use the I/O command under the protected mode,whitch is helpful for the protection of ports. If it will visit the different Privilege Level while executing a program,the CPL,RPL,DPL and the IOPL must be checkout strictly first.And it will do something with the change of stack in order to protect different hierarchy of program.

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