Global Descriptor Table: Difference between revisions

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m Make flag names consistent in the explanations
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** For data selectors: Direction bit. If clear ('''0''') the segment grows up. If set ('''1''') the segment [[Expand_Down|grows down]], ie. the '''Offset''' has to be greater than the '''Limit'''.
** For data selectors: Direction bit. If clear ('''0''') the segment grows up. If set ('''1''') the segment [[Expand_Down|grows down]], ie. the '''Offset''' has to be greater than the '''Limit'''.
** For code selectors: Conforming bit.
** For code selectors: Conforming bit.
*** If clear ('''0''') code in this segment can only be executed from the ring set in '''Privl'''.
*** If clear ('''0''') code in this segment can only be executed from the ring set in '''DPL'''.
*** If set ('''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''' field 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 where '''Privl''' is 2, while code in ring 2 and 3 can. Note that the privilege level remains the same, ie. a far-jump from ring 3 to a segment with a '''Privl''' of 2 remains in ring 3 after the jump.
*** If set ('''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 '''DPL''' field 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 where '''DPL''' is 2, while code in ring 2 and 3 can. Note that the privilege level remains the same, ie. a far-jump from ring 3 to a segment with a '''DPL''' of 2 remains in ring 3 after the jump.
* '''RW:''' Readable bit/Writable bit.
* '''RW:''' Readable bit/Writable bit.
** For code segments: Readable bit. If clear ('''0'''), read access for this segment is not allowed. If set ('''1''') read access is allowed. Write access is never allowed for code segments.
** For code segments: Readable bit. If clear ('''0'''), read access for this segment is not allowed. If set ('''1''') read access is allowed. Write access is never allowed for code segments.
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* '''G:''' Granularity flag, indicates the size the '''Limit''' value is scaled by. If clear ('''0'''), the '''Limit''' is in 1 Byte blocks (byte granularity). If set ('''1'''), the '''Limit''' is in 4 KiB blocks (page granularity).
* '''G:''' Granularity flag, indicates the size the '''Limit''' value is scaled by. If clear ('''0'''), the '''Limit''' is in 1 Byte blocks (byte granularity). If set ('''1'''), the '''Limit''' is in 4 KiB blocks (page granularity).
* '''DB:''' Size flag. If clear ('''0'''), the descriptor defines a 16-bit protected mode segment. If set ('''1''') it defines a 32-bit protected mode segment. A GDT can have both 16-bit and 32-bit selectors at once.
* '''DB:''' Size flag. If clear ('''0'''), the descriptor defines a 16-bit protected mode segment. If set ('''1''') it defines a 32-bit protected mode segment. A GDT can have both 16-bit and 32-bit selectors at once.
* '''L:''' Long-mode code flag. If set ('''1'''), the descriptor defines a 64-bit code segment. When set, '''Sz''' should always be clear. For any other type of segment (other code types or any data segment), it should be clear ('''0''').
* '''L:''' Long-mode code flag. If set ('''1'''), the descriptor defines a 64-bit code segment. When set, '''DB''' should always be clear. For any other type of segment (other code types or any data segment), it should be clear ('''0''').


== System Segment Descriptor ==
== System Segment Descriptor ==
Retrieved from "https://osdev.wiki/wiki/Global_Descriptor_Table"