SECTION .text ; cannot use .data

struc GDTEntry ; spell:disable-line
    .limitl resw 1
    .basel resw 1
    .basem resb 1
    .attribute resb 1
    .flags__limith resb 1
    .baseh resb 1
endstruc

gdt_attr:
    .present              equ 1 << 7
    .ring1                equ 1 << 5
    .ring2                equ 1 << 6
    .ring3                equ 1 << 5 | 1 << 6
    .user                 equ 1 << 4
;user
    .code                 equ 1 << 3
;   code
    .conforming           equ 1 << 2
    .readable             equ 1 << 1
;   data
    .expand_down          equ 1 << 2
    .writable             equ 1 << 1
    .accessed             equ 1 << 0
;system
;   legacy
    .tssAvailabe16        equ 0x1 ; spell:disable-line
    .ldt                  equ 0x2
    .tssBusy16            equ 0x3
    .call16               equ 0x4
    .task                 equ 0x5
    .interrupt16          equ 0x6
    .trap16               equ 0x7
    .tssAvailabe32        equ 0x9 ; spell:disable-line
    .tssBusy32            equ 0xB
    .call32               equ 0xC
    .interrupt32          equ 0xE
    .trap32               equ 0xF
;   long mode
    .ldt32                equ 0x2
    .tssAvailabe64        equ 0x9 ; spell:disable-line
    .tssBusy64            equ 0xB
    .call64               equ 0xC
    .interrupt64          equ 0xE
    .trap64               equ 0xF

gdt_flag:
    .granularity equ 1 << 7
    .available equ 1 << 4
;user
    .default_operand_size equ 1 << 6
;   code
    .long_mode equ 1 << 5
;   data
    .reserved equ 1 << 5

gdtr:
    dw gdt.end + 1  ; size
    dq gdt          ; offset

gdt:
.null equ $ - gdt
    dq 0

; All GTD addresses are multiples of 8,
; and thus end in three zero bits.
;
; Also note that all segments overlap.
; We only need the GDT for a short time,
; so we use the simplest possible layout.
.pm32_code equ $ - gdt
    istruc GDTEntry
        at GDTEntry.limitl, dw 0xFFFF
        at GDTEntry.basel, dw 0
        at GDTEntry.basem, db 0
        at GDTEntry.attribute, db gdt_attr.present | gdt_attr.user | gdt_attr.code | gdt_attr.readable
        at GDTEntry.flags__limith, db 0xF | gdt_flag.granularity | gdt_flag.default_operand_size
        at GDTEntry.baseh, db 0
    iend

.pm32_data equ $ - gdt
    istruc GDTEntry
        at GDTEntry.limitl, dw 0xFFFF
        at GDTEntry.basel, dw 0
        at GDTEntry.basem, db 0
        at GDTEntry.attribute, db gdt_attr.present | gdt_attr.user | gdt_attr.writable
        at GDTEntry.flags__limith, db 0xF | gdt_flag.granularity | gdt_flag.default_operand_size
        at GDTEntry.baseh, db 0
    iend

.pm16_code equ $ - gdt
    istruc GDTEntry
        at GDTEntry.limitl, dw 0xFFFF
        at GDTEntry.basel, dw 0
        at GDTEntry.basem, db 0
        at GDTEntry.attribute, db gdt_attr.present | gdt_attr.user | gdt_attr.code | gdt_attr.readable
        at GDTEntry.flags__limith, db 0xF
        at GDTEntry.baseh, db 0
    iend

.pm16_data equ $ - gdt
    istruc GDTEntry
        at GDTEntry.limitl, dw 0xFFFF
        at GDTEntry.basel, dw 0
        at GDTEntry.basem, db 0
        at GDTEntry.attribute, db gdt_attr.present | gdt_attr.user | gdt_attr.writable
        at GDTEntry.flags__limith, db 0xF
        at GDTEntry.baseh, db 0
    iend

.end equ $ - gdt