---
title: "gdt.h"
summary: "definitions to handle segments (_Global Descriptor Table_)"
---

Segmentation is deprecated and is only needed for backwards compatibility.

OSDev Wiki: [Global Descriptor Table](https://wiki.osdev.org/GDT)

# `gdt_selector_E` - enum
- **Null**
- **Kernel Code** - Readable
- **Kernel Data** - Readable + Writable
- **User Null**
- **User Code** - Readable
- **User Data** - Readable + Writable

# `gdt_descriptor_T` - struct [packed]

| Name   | Type     | Description                                         |
|--------|----------|-----------------------------------------------------|
| size   | uint16_t | The tables size in bytes (-1)                       |
| offset | uint64_t | The virtual address, where the table lies in memory |


# `gdt_entry_T` - struct [packed]
| Name         | Type     | Description                                                  |
|--------------|----------|--------------------------------------------------------------|
| limit0       | uint16_t | Can be ignored in long mode                                  |
| base0        | uint16_t | Can be ignored in long mode                                  |
| base1        | uint8_t  | Can be ignored in long mode                                  |
| access       | uint8_t  | Specifies permissions (details in osdev wiki)                |
| limit1_flags | uint8_t  | The first 4 bits can be ignored and the last 4 specify flags |
| base2        | uint8_t  | Can be ignored in long mode                                  |

# `gdt_T` - struct [packed / page aligned]
| Name        | Type        | Description                              |
|-------------|-------------|------------------------------------------|
| null        | gdt_entry_T | The entry for `GDT_SELECTOR_NULL`        |
| kernel_code | gdt_entry_T | The entry for `GDT_SELECTOR_KERNEL_CODE` |
| kernel_data | gdt_entry_T | The entry for `GDT_SELECTOR_KERNEL_DATA` |
| user_null   | gdt_entry_T | The entry for `GDT_SELECTOR_USER_NULL`   |
| user_code   | gdt_entry_T | The entry for `GDT_SELECTOR_USER_CODE`   |
| user_data   | gdt_entry_T | The entry for `GDT_SELECTOR_USER_DATA`   |

# `g_default_gdt` - global variable
The systems GDT.

# `gdt_init` - function (void)
Populates and loads [g_default_gdt](https://nerdcult.net/projects/noxos/docs/codebase/platform/gdt.h/#g_default_gdt---global-variable).
This will also set all the data segment registers to 0x10 (Kernel Data) and `cs` to 0x08 (Kernel Code).