-1 
[org 0x7C00]

 
[bits 16]

mov bp, 0x0500
mov sp, bp

  
 
[bits 16]

 
load_bios:
   
    ; Save the number of registers to load for later
    push cx

    ; For the ATA Read bios utility, the value of ah must be 0x02
    ; See the BIOS article from Chapter 1.2 for more info
    mov ah, 0x02
    mov al, 0x01
    mov cl, 0x02
    mov bx, 0x7e00

    mov ch, 0x00        ; Cylinder goes in ch
    mov dh, 0x00        ; Cylinder head goes in dh
     ; dl is already stored in dl
    int 0x13

  
; And elevate our CPU to 32-bit mode
;call elevate_bios
elevate_bios:




    cli
 
    lgdt [gdt_32_descriptor]

    mov eax, cr0
    or eax, 0x00000001
    mov cr0, eax

    jmp 0x8:init_pm ;code segment is the second segment 
                    ;

jmp $
[bits 16]


  
gdt_32_descriptor:
    dw gdt_32_end - gdt_32_start - 1        ; Size of GDT, one byte less than true size
    dd gdt_32_start                         ; Start of the 32 bit gdt

 
gdt_32_start:
; Define the null sector for the 32 bit gdt
; Null sector is required for memory integrity check
gdt_32_null:
    dd 0x00000000           ; All values in null entry are 0
    dd 0x00000000           ; All values in null entry are 0

; Define the code sector for the 32 bit gdt
gdt_32_code:
  

    dw 0xFFFF           ; Limit (bits 0-15)
    dw 0x0000           ; Base  (bits 0-15)
    db 0x00             ; Base  (bits 16-23)
    db 0b10011010       ; 1st Flags, Type flags
    db 0b11001111       ; 2nd Flags, Limit (bits 16-19)
    db 0x00             ; Base  (bits 24-31)

; Define the data sector for the 32 bit gdt
gdt_32_data:


    dw 0xFFFF           ; Limit (bits 0-15)
    dw 0x0000           ; Base  (bits 0-15)
    db 0x00             ; Base  (bits 16-23)
    db 0b10010010       ; 1st Flags, Type flags
    db 0b11001111       ; 2nd Flags, Limit (bits 16-19)
    db 0x00             ; Base  (bits 24-31)

gdt_32_end:
 
; Define helpers to find pointers to Code and Data segments
code_seg:                           equ gdt_32_code - gdt_32_start
data_seg:                           equ gdt_32_data - gdt_32_start
; Infinite Loop
bootsector_hold:
jmp $               ; Infinite loop
 

    [bits 32]
    init_pm:

    ;segment register contains the gdt sectors (it states where the sgment starts and sizw)
    ; when we address it takes an offset from the address that the current sector 
    ;starts )
    mov ax, data_seg
    mov ds, ax
    mov ss, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    ; Since the stack pointers got messed up in the elevation process, and we
    ; want a fresh stack, we need to reset them now.
    mov ebp, 0x90000
    mov esp, ebp

    ; Go to the second sector with 32-bit code
    jmp begin_protected
 
 
times 510 - ($ - $$) db 0x00

; Magic number
dw 0xAA55


; BEGIN SECOND SECTOR. THIS ONE CONTAINS 32-BIT CODE ONLY

bootsector_extended:
begin_protected:
 
;routine
;;;;;;;;;;;;;;;;;;;;;;;;;
mov ebx  , 0x8b000         ;;;  start  ;;;;;;;;;;;;;;;;;;;;;;;;;
mov ah , style_wb
loop1:

mov al , 0x43
mov [ebx] , ax 
add ebx , 2  
cmp ebx , 0xb8000 + 80*2  ;;;;;; end ;;;;;;;;;;;;;;;;;;
jl loop1
;;;;;;;;;;;;;;;;;;;;;;;;
;routine





jmp $       ; Infinite Loop
 
 
[bits 32]
 

 
; Fill with zeros to the end of the sector
times 512 - ($ - bootsector_extended) db 0x00

在这个 gdt 中，我将代码和数据段设置为从地址 0 开始并占用整个 4gb 的内存。我猜该表存在于这个内存中的某个地方（我不太确定，但我猜它存储在前 512+0x7c00 个字节中，在我更改为 32 位模式之前由 bios 加载）。如果这是真的，我可以达到此地址（因为数据段设置为从地址 0 开始）

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