; GDT
gdt_start:

gdt_null:  ; the mandatory null descriptor
  dd 0x0  ; ’dd’ means define double word (i.e. 4 bytes)
  dd 0x0

gdt_code: ; the code segment descriptor
  ; base=0x0, limit=0xfffff,
  ; 1st flags: (present)1 (privilege)00 (descriptor type)1 -> 1001b
  ; type flags: (code)1 (conforming)0 (readable)1 (accessed )0 -> 1010b
  ; 2nd flags: (granularity)1 (32-bit default)1 (64-bit seg)0 (AVL)0 -> 1100b
  dw 0xffff     ; Limit (bits 0-15)
  dw 0x0        ; Base (bits 0-15)
  db 0x0        ; Base (bits 16 -23)
  db 10011010b  ; 1st flags, type flags
  db 11001111b  ; 2nd flags, Limit (bits 16-19)
  db 0x0        ; Base (bits 24 -31)

gdt_data: ;the  data segment descriptor
  ; Same as code segment except for the type flags:
  ; type flags: (code)0 (expand down)0 (writable)1 (accessed)0 -> 0010b
  dw 0xffff    ; Limit (bits 0-15)
  dw 0x0       ; Base (bits 0-15)
  db 0x0       ; Base (bits 16 -23)
  db 10010010b ; 1st flags, type flags
  db 11001111b ; 2nd flags, Limit (bits 16-19)
  db 0x0       ; Base (bits 24 -31)

gdt_end:       ; The reason for putting a label at the end of the
               ; GDT is so we can have the assembler calculate
               ; the size of the GDT for the GDT decriptor (below)

; GDT descriptor
gdt_descriptor:
  dw gdt_end - gdt_start - 1  ; Size of our GDT, always less one
                              ; of the true size
  dd gdt_start                ; Start address of our GDT

; Define some handy constants for the GDT segment descriptor offsets, which
; are what segment registers must contain when in protected mode. For example,
; when we set DS = 0x10 in PM , the CPU knows that we mean it to use the
; segment described at offset 0x10 (i.e. 16 bytes) in our GDT, which in our
; case is the DATA segment (0x0 -> NULL; 0x08 -> CODE; 0x10 -> DATA)
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start