HalGenerateGDTDesc

HalGenerateGDTDesc Function

Description

This function generates a valid GDT descriptor structure for later use

Syntax

gdt_descriptor HalGenerateGDTDesc(dword baseAddress, 
                                  dword segmentLimit, 
                                  gdtflags gdtFlags
);

Parameters

baseAddress

Base address of the descriptor

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segmentLimit

The ammount of memory the decriptor covers !WARNING! Only first 28 bytes of this value are used ! If you want to use a value larger then 0x0FFFFFFF, specify gdt_granularity in gdtFlags (see gdtFlags)

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gdtFlags

The bitflags used for the descriptor

gdt_accessBit
Do not specify this flag, it's meant for internal CPU usage.

gdt_isDescriptorWriteable
Data descriptors: If this flag is set, the descriptor is read/write. Otherwise, it's read-only.
Code descriptors: If this flag is set, the descriptor is readable. Otherwise, it isn't.

gdt_expandDown
If this flag is set, the descriptor expands downwards (eg. for use as a stack descriptor). Otherwise, the descriptor expands upwards. Only valid for data descriptors, it is treated as gdt_isConforming on code descriptors.

gdt_isConforming
If this flag is set, lower privilege levels then specified can use this selector, eg if gdt_ring2 is specified, the selector will be accessible from rings 3 and 2 only. If not, it is only accessible from the specified ring. Only valid for code descriptors, it is treated as gdt_expandDown on data descriptors.

gdt_isExecutable
If gdt_isCodeOrDataDescriptor is set: If this flag is set, the descriptor is a code descriptor. Otherwise it's a data descriptor.
If gdt_isCodeOrDataDescriptor is not set: If this flag is set, the descriptor is executable. Otherwsie, it isn't.

gdt_ring0
This flag specifies the descriptor is accessible from ring 0. Not valid with gdt_ring1, gdt_ring2 or gdt_ring3

gdt_ring1
This flag specifies the descriptor is accessible from ring 1. Not valid with gdt_ring0, gdt_ring2 or gdt_ring3

gdt_ring2
This flag specifies the descriptor is accessible from ring 2. Not valid with gdt_ring0, gdt_ring1 or gdt_ring3

gdt_ring3
This flag specifies the descriptor is accessible from ring 3. Not valid with gdt_ring0, gdt_ring1 or gdt_ring2

gdt_segmentIsInMemory
This flag must be set, otherwise the descriptor is disabled and ignored.

gdt_reservedOSUse
This flag is reserved for OS use. Do not specify this flag.

gdt_segmentIs32Bit
If this flag is set, the descirptor is a 32 bit descriptor. Otherwise it's a 16 bit descriptor.

gdt_granularity
If this flag is set, segmentLimit is multiplied by 4096 (4k, the size of a page)

Return Value

This function always returns a GDT descriptor structure.