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; GDT global descriptor table
gdt_start:
; mandatory null entry
gdt_null:
dd 0x0
dd 0x0
; code segment
; base=0x0, limit=0xfffff
; flags: present (not paged), ring 0, executable, direction bit
; conforming, writable, not accessed
; granularity: 4kb pages, 32-bit mode, no 64-bit segment, AVL would
; be for our own extensions
gdt_code:
dw 0xffff ; limit (bits 0-15)
dw 0x0 ; base (bits 0-15)
db 0x0 ; base (bits 16-23)
db 10011010b ; flags
db 11001111b ; flags, limit (bits 16-19)
db 0x0 ; base (bit 24-31)
; flat model, same as code segment, but flags are
; flags: ring 0,
gdt_data:
dw 0xffff ; limit (bits 0-15)
dw 0x0 ; base (bits 0-15)
db 0x0 ; base (bits 16-23)
db 10010010b ; flags
db 11001111b ; flags, limit (bits 16-19)
db 0x0 ; base (bit 24-31)
gdt_end:
gdt_descriptor:
dw gdt_end -gdt_start - 1 ; size
dd gdt_start ; start address of the GDT
; constants representing the segment bases
CODE_SEGMENT equ gdt_code - gdt_start
DATA_SEGMENT equ gdt_data - gdt_start
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