gdt_entry Struct Reference

Data Fields
uint16_t	limit_low16
	Low 16 bits of the limit.
uint16_t	base_low16
	Low 16 bits of the base.
uint8_t	base_mid8
	Middle 8 bits of the base.
uint8_t	access
uint8_t	limit_flags
uint8_t	base_high8
	High 8 bits of the base.

Detailed Description

Represents an entry in the GDT.

Field Documentation

access

uint8_t gdt_entry::access

Access byte. Gives info about the descriptor.
Format:
Bit 7: Present. Must be 1 for all valid selectors.
Bits 6-5. Privilege. Contains the ring level for the selector. 0 for kernel mode, 3 for user mode.
Bit 4. Descriptor type. Must be set for code/data segments and cleared for system segments like the TSS.
Bit 3. Executable. If this bit is set, it is a code selector, otherwise a data selector.
Bit 2. Direction/Conforming. Too complex to explain, should be set to 0.
Bit 1. Readable/Writable. For code sels, this bit sets whther you can use it like a read-only data segment. For data sels, it sets whether the selector is writable.
Bit 0. Acessed bit. Set to 0.

limit_flags

uint8_t gdt_entry::limit_flags

High nibble of this contains two flags, and the lower niblle contains the high 4 bits of the limit.
The flags are:
Bit 3. Granularity. 0 for byte granularity, 1 for 4 KB granularity.
Bit 2. Size. 0 for 16 bit protected mode, 1 for 32 bit protected mode.
Bits 1-0. Unused. Set to 0.

---

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

• kernel/cpu/i386/gdt.c