gdt.asm

; 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 =0 x0 , limit =0 xfffff ,
; 1 st flags : ( present )1 ( privilege )00 ( descriptor type )1 -> 1001 b
; type flags : ( code )1 ( conforming )0 ( readable )1 ( accessed )0 -> 1010 b
; 2 nd flags : ( granularity )1 (32 - bit default )1 (64 - bit seg )0 ( AVL )0 -> 1100 b
dw 0xffff
; Limit ( bits 0 -15)
dw 0x0
; Base ( bits 0 -15)
db 0x0
; Base ( bits 16 -23)
db 10011010b ; 1 st flags , type flags
db 11001111b ; 2 nd 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 -> 0010 b
dw 0xffff
; Limit ( bits 0 -15)
dw 0x0
; Base ( bits 0 -15)
db 0x0
; Base ( bits 16 -23)
db 10010010b ; 1 st flags , type flags
db 11001111b ; 2 nd 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 descriptior
gdt_descriptor :
dw gdt_end - gdt_start - 1
dd gdt_start
; Size of our GDT , always less one
; of the true size
; 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 = 0 x10 in PM , the CPU knows that we mean it to use the
; segment described at offset 0 x10 ( i.e. 16 bytes ) in our GDT , which in our
; case is the DATA segment (0 x0 -> NULL ; 0 x08 -> CODE ; 0 x10 -> DATA )
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start