section .rodata

gdt_flat_slice:
  dw GDT_FLAT_LEN
  dd gdt_flat

global gdt_flat_slice

; Segment descriptor layout
; | Range (bits) | Field         |
; |--------------|---------------|
; |         0-16 | limit         |
; |        16-32 | base          |
; |        32-40 | base cont.    |
; |        40-48 | access        |
; |        48-52 | limit cont.   |
; |        52-56 | flags         |
; |        56-64 | base cont.    |
;
; Flags
; - 0: reserved
; - 1: long-mode code segment
; - 2: size
;   - unset: 16-bit
;   - set: 32-bit
; - 3: granularity
;   - unset: limit is measured in bytes
;   - set: limit is measured in 4KiB pages
;
; Access
; - 0: accessed
;   - unset: CPU will set it when the segment is accessed
; - 1: readable / writable
;   - data segments: is segment writable (data segments are always readable)
;   - code segments: is segment readable (code segments are never writable)
; - 2: direction / conforming
;   - data segments: whether segment grows down
;   - code segments: whether this can be executed from a lower-privilege ring
; - 3: executable
;   - unset: this is a data segment
;   - set: this is a code segment
; - 4: descriptor type
;   - unset: this is a task state segment
;   - set: this is a data or code segment
; - 5-6: privilege level (ring number)
; - 7: present (must be set)
;
align 8
gdt_flat:
  ; First GDT entry must be 0.
  dq 0

; 32-bit code segment.
; Bytes 0x0000 - 0xffff.
.segment_code_32:
  db 0xff, 0xff, \
     0x00, 0x00, \
     0x00,       \
     10011011b,  \
     01000000b,  \
     0x00

; 16-bit code segment, to use if we want to switch back to real mode.
; Bytes 0x0000 - 0xffff.
.segment_code_16:
  db 0xff, 0xff, \
     0x00, 0x00, \
     0x00,       \
     10011011b,  \
     00000000b,  \
     0x00

; Data segment.
; Pages 0x000000 - 0x0fffff, which covers the entire 32-bit address space (start of 0xfffff-th page
; is 0xfffff * 4096 = 0xfffff000, end of page exclusive is 0xfffff000 + 4096 = 0x100000000).
.segment_data:
  db 0xff, 0xff, \
     0x00, 0x00, \
     0x00,       \
     10010011b,  \
     11001111b,  \
     0x00

global gdt_flat

GDT_FLAT_LEN equ ($ - gdt_flat)

GDT_FLAT_IDX_CODE_32 equ (gdt_flat.segment_code_32 - gdt_flat)
global GDT_FLAT_IDX_CODE_32

GDT_FLAT_IDX_CODE_16 equ (gdt_flat.segment_code_16 - gdt_flat)
global GDT_FLAT_IDX_CODE_16

GDT_FLAT_IDX_DATA equ (gdt_flat.segment_data - gdt_flat)
global GDT_FLAT_IDX_DATA