GDT::SegmentDescriptor Union Reference

Describes the structure of segment descriptors. More...

#include <gdt.h>

Collaboration diagram for GDT::SegmentDescriptor:

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, Size size)
	Constructor for a code/data GDT entry. More...

Public Attributes
struct {
uint64_t   __pad0__: 40
	Ignored (set via limit_low and base_low )
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.
bool   code: 1
	Has to be set to true
uint64_t   __pad1__: 9
	Ignored (set via privilege_level ... available)
Size   operation_size: 2
	Default address width (custom field bit)
uint64_t   __pad2__: 0
	Remainder ignored (set via base_high)
};
	Fields specific for Code Segment (for debugging purposes) More...
struct {
uint64_t   __pad0__: 40
	Ignored (set via limit_low and base_low)
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.
bool   notData: 1
	Has to be cleared (false)
uint64_t   __pad1__: 9
	Ignored (set via privilege_level ... available)
uint64_t   reserved: 1
	Reserved, always set to 0!
bool   big: 1
	Size of the stack pointer (false = 16 bit, true = 32 bit) More...
uint64_t   __pad2__: 0
	Remainder ignored.
};
	Fields specific for Data Segment (for debugging purposes) More...
struct {
uint64_t   __pad0__: 40
	Ignored (set voa limit_low and base_low)
enum Gate   gate: 3
	TSS or LDT entry.
bool   not16bit: 1
	false for 16-bit, true for 32-bit/64-bit
uint64_t   __pad1__: 0
	Remainder ignored.
};
	Fields specific to the Task State Segment. More...
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: 4
	Meaning of those 4 bits depends on descriptor_type below.
DescriptorType	descriptor_type: 1
	Descriptor 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 true
uint64_t	limit_high: 4
	Most-significant bits of segment size.
bool	available: 1
	Bit which can be used for other purposes (in software)
uint64_t	custom: 2
	Meaning of those 2 bits relate to descriptor_type and type.
Granularity	granularity: 1
	Unit used as granularity for the segment limit.
uint64_t	base_high: 8
	most-significant bits of base address

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

AAPMv2, 4.7 Legacy Segment Descriptors

Constructor & Destructor Documentation

SegmentDescriptor() [1/2]

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

inlineconstexpr

!< 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, Size  size  )

inlineconstexpr

Constructor for a code/data GDT entry.

Parameters

base	Base Address of segment
limit	Size of segment
code	Code or data segment
ring	Privilege level
size	Address width

Member Data Documentation

struct { ... }

Fields specific for Code Segment (for debugging purposes)

See also

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

struct { ... }

Fields specific for Data Segment (for debugging purposes)

See also

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

struct { ... }

Fields specific to the Task State Segment.

See also

ISDMv3, 7.2.2; TSS Descriptor

__pad0__

uint64_t GDT::SegmentDescriptor::__pad0__

Ignored (set via limit_low and base_low )

Ignored (set voa limit_low and base_low)

Ignored (set via limit_low and base_low)

__pad1__

uint64_t GDT::SegmentDescriptor::__pad1__

Ignored (set via privilege_level ... available)

Remainder ignored.

__pad2__

uint64_t GDT::SegmentDescriptor::__pad2__

Remainder ignored (set via base_high)

Remainder ignored.

big

bool GDT::SegmentDescriptor::big

Size of the stack pointer (false = 16 bit, true = 32 bit)

Warning

Has a different meaning in case expand_down is set to 1.

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

• machine/gdt.h