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Introduction

This document explains all the code in the file src/boot_sector/gdt.asm.

What does it mean?

GDT stands for Global Descriptor Table, basically it is a binary structure. It contains entries telling the CPU about memory segments. A similar Interrupt Descriptor Table exists containing task and interrupt descriptors.

What does it do ?

As we remember, in real mode (16 bits), the segmentation is managed by the segment registers (cs, ds, es, fs and gs). Now it is managed by the segment stored in the GDT.

The GDT should be seen as a large table with segment descriptors as entries.

Each entry has a size of 8 bytes, remember that the first one is always filled with zeros, it looks like this

first_segment:
    dd 0x0
    dd 0x0

Every other entries should look like this:

entry_example: 
    dw 0xffff       ; Segment limit first 0-15 bits
    dw 0            ; Base first 0-15 bits
    db 0            ; Base 16-23 bits
    db 0x9a         ; Access byte
    db 11001111b    ; High 4 bit flags and the low 4 bit flags
    db 0            ; Base 24-31 bits

Loading the GDT

To load the GDT, there is an instruction named lgdt, it means Load Global Descriptor Table. lgdt requires a 6 bytes structure, in this code we named it gdt_descriptor, as we can see below:

lgdt [gdt_descriptor]

Usage example

As example you can see in src/boot_sector/switch_pm.asm:

jmp CODE_SEG:init_pm

Here, CODE_SEG is not a reference to the segment itself. Actually, it refers to the segment descriptor entry in the GDT. Moreover, in this part of code we were not in the segment CODE, it will make us using a different segment. It is called a far jump.