segment descriptor 格式备忘

The link: http://wiki.osdev.org/Global_Descriptor_Table

The bit fields are:
Pr: Present bit. This must be 1 for all valid selectors.
Privl: Privilege, 2 bits. Contains the ring level, 0 = highest (kernel), 3 = lowest (user applications).
Ex: Executable bit. If 1 code in this segment can be executed, ie. a code selector. If 0 it is a data selector.
DC: Direction bit/Conforming bit.
Direction bit for data selectors: Tells the direction. 0 the segment grows up. 1 the segment grows down, ie. the offset has to be greater than the base.
Conforming bit for code selectors:
If 1 code in this segment can be executed from an equal or lower privilege level. For example, code in ring 3 can far-jump to conforming code in a ring 2 segment. The privl-bits represent the highest privilege level that is allowed to execute the segment. For example, code in ring 0 cannot far-jump to a conforming code segment with privl==0x2, while code in ring 2 and 3 can. Note that the privilege level remains the same, ie. a far-jump form ring 3 to a privl==2-segment remains in ring 3 after the jump.
If 0 code in this segment can only be executed from the ring set in privl.
RW: Readable bit/Writable bit.
Readable bit for code selectors: Whether read access for this segment is allowed. Write access is never allowed for code segments.
Writable bit for data selectors: Whether write access for this segment is allowed. Read access is always allowed for data segments.
Ac: Accessed bit. Just set to 0. The CPU sets this to 1 when the segment is accessed.
Gr: Granularity bit. If 0 the limit is in 1 B blocks (byte granularity), if 1 the limit is in 4 KiB blocks (page granularity).
Sz: Size bit. If 0 the selector defines 16 bit protected mode. If 1 it defines 32 bit protected mode. You can have both 16 bit and 32 bit selectors at once.