GDT::SegmentDescriptor Union Reference

Describes the structure of segment descriptors. More...

#include <machine/gdt.h>

Public Member Functions
constexpr	SegmentDescriptor (uint64_t val=0)
	!< Merged value; useful for debugging More...
constexpr	SegmentDescriptor (uintptr_t base, uint32_t limit, bool code, int ring, Mode mode)
	Constructor for a 16/32 bit code/memory GDT entry. More...

Public Attributes
struct {
uint64_t	limit_low: 16
	Least-significant bits of segment size (influenced by granularity!)
uint64_t	base_low: 24
	Least-significant bits of base address.
uint64_t	type: 3
	Meaning of those 3 bits depends on segment_type below.
enum Type	segment_type: 2
	Segment type (influences the meaning of the 3 bits above)
uint64_t	privilege_level: 2
	Ring for this segment.
bool	present: 1
	Entry is valid iff set to 1.
uint64_t	limit_high: 4
	Most-significant bits of segment size.
bool	available: 1
	One bit to use for other purposes.
uint64_t	custom: 2
	Meaning of those 2 bits depends on segment_type below.
Granularity	granularity: 1
	Unit used as granularity for the segment limit.
uint64_t	base_high: 8
	most-significant bits of base address
};
struct {
uint64_t	__pad0__: 40
	Ignored.
bool	code_accessed: 1
	If set, the code segment was used since the last reset of this value.
bool	readable: 1
	If set the code is readable (otherwise only executable)
bool	conforming: 1
	If set, the execution of code from this segment is only allowed when running at a privilege of numerically less than or equal to privilege_level (i.e. the executor has the same or higher privileges). However, the executor's privileges remain unchanged. For nonconforming code segments (i.e., conforming is set to 0), execution is allowed only if the privileges are equal. Execution will cause a GPF in case of privilege violation.
uint64_t	__pad1__: 10
	Ignored.
bool	is64bit: 1
	64bit code segment More...
Size	operation_size: 1
	Default address width.
uint64_t	__pad2__: 0
	Remainder ignored.
};
	Fields specific for Code Segment. More...
struct {
uint64_t	__pad0__: 40
	Ignored.
bool	data_accessed: 1
	If set, the data segment was used since the last reset of this value.
bool	writeable: 1
	If set, data is writable (otherwise read only)
bool	expand_down: 1
	Growing direction for dynamically growing segments.
uint64_t	__pad1__: 10
	Ignored.
uint64_t	reserved: 1
	Reserved, always set to 0!
Size	big: 1
	Size of the stack pointer. More...
uint64_t	__pad2__: 0
	Remainder ignored.
};
	Fields specific for Data Segment. More...
struct {
uint64_t	__pad0__: 40
	Ignored.
enum Gate	gate: 3
	TSS or LDT entry.
uint64_t	__pad1__: 0
	Remainder ignored.
};
	Fields specific to the Task State Segment. More...
struct {
uint64_t	base64_high: 32
	Most-significant part of 64bit TSS entries.
uint64_t	__pad0__: 8
	Reserved.
uint64_t	zero: 5
	Zero.
uint64_t	__pad1__: 0
	Remainder ignored.
};
	Fields specific to the second part of the Task State Segment on x64. More...
uint64_t	value

Detailed Description

Describes the structure of segment descriptors.

A data structure that contains size, position, access rights, and purpose of any segment. Segment descriptors are used in both the GDT, as well as in LDTs.

See also

ISDMv3, 3.4.5; Segment Descriptors

Constructor & Destructor Documentation

SegmentDescriptor() [1/2]

constexpr GDT::SegmentDescriptor::SegmentDescriptor ( uint64_t  val = 0 )

inline

!< Merged value; useful for debugging

Constructor for a specific value

SegmentDescriptor() [2/2]

constexpr GDT::SegmentDescriptor::SegmentDescriptor ( uintptr_t  base, uint32_t  limit, bool  code, int  ring, Mode  mode  )

inline

Constructor for a 16/32 bit code/memory GDT entry.

Parameters

base	Base Address
limit	Size
code	Code or data segment
ring	Privilege level
mode	Operation mode (real/protected/long)

Member Data Documentation

@32

struct { ... }

Fields specific for Code Segment.

See also

ISDMv3, 3.4.5.1; Code- and Data-Segment Descriptor Types

@34

struct { ... }

Fields specific for Data Segment.

See also

ISDMv3, 3.4.5.1; Code- and Data-Segment Descriptor Types

@36

struct { ... }

Fields specific to the Task State Segment.

See also

ISDMv3, 7.2.2; TSS Descriptor

@38

struct { ... }

Fields specific to the second part of the Task State Segment on x64.

See also

ISDMv3, 7.2.3; TSS Descriptor in 64bit mode

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The documentation for this union was generated from the following file:

• machine/gdt.h