GDT assembly code

global _gdt_install

_gdt_install:
    cli
    lgdt [gdt]          ; load GDT into GDTR
    mov ax, 0x10
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    ;mov ss, ax

    jmp 0x08:return

return:
    ret

; null descriptor

gdt_start:
    dd 0x00
    dd 0x00

; code descriptor: indicates readable and writable segment, and is at ring 0

    dw 0xFFFF       ; limit low: segment limit; each segment is 64K
    dw 0x00         ; base low (starting address of segment): code selector can access every byte between 0x00 and 0xFFFF
    db 0x00         ; base middle and base low is 3 bytes (bit 0-23 of base address)
    db 10011010b    ; access byte: bits 40-47 (change bits 40 and 47 when using virtual memory)
    db 11001111b    ; granularity bit: bits 48-55
    db 0x00         ; base high (bits 24-32 of the base address)

; same as code descriptor, except for bit 43, indicating this is a code descriptor

    dw 0xFFFF       ; limit low
    dw 0x00         ; base low
    db 0x00         ; base middle
    db 10011010b    ; access byte
    db 11001111b    ; granularity byte
    db 0x00         ; base high

gdt_end:

gdt:
    dw gdt_end - gdt_start - 1 ; limit (size of GDT)
    dd gdt_start               ; base address of GDT