Writing a Simple Operating System — from Scratch

CHAPTER 4. ENTERING 32-BIT PROTECTED MODE 36

structure and how the flags are more conveniently defined using literal binary numbers,
that are suffixed withb:

; 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 descriptior

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

4.4 Making the Switch

Once both the GDT and the GDT descriptor have been prepared within our boot sector,
we are ready to instruct the CPU to switch from 16-bit real mode into 32-bit protected
mode.