1 [bits 32]
2 start_32:
3 mov ax,0x10
4 mov ds,ax
5 mov word [gdtr],gdt_end-gdt
6 mov dword [gdtr+2],gdt
7 mov word [idtr],idt_end-idt
8 mov dword [idtr+2],idt
9 lgdt [gdtr]
10 lidt [idtr]
11 mov ax,0x10
12 mov ds,ax
13 mov es,ax
14 mov ss,ax ; stack segment = data segment = code seg
15 mov sp,stack_top
16 mov ax,0x18
17 mov gs,ax
18
19 call clr_scr
20 call welcome
21
22 jmp $
23
24 welcome:
25 mov esi,str_welcome
26 sub edi,edi
27 mov ah,0ch
28 rp_welcome:
29 mov al,[esi]
30 test al,al ; al & al = 0 ?
31 jz end_welcome
32 mov [gs:edi],ax
33 inc esi
34 add edi,2
35 jmp rp_welcome
36 end_welcome:
37 ret
38
39 clr_scr:
40 xor ah,ah
41 mov al,' '
42 mov ecx,2000
43 sub edi,edi
44 rp_clr:
45 mov [gs:edi],ax
46 add edi,2
47 dec ecx
48 jne rp_clr
49 ret
50
51
52
53 ;---------------------------------------------------------------------
54 ; data segment, ds:esi, r/w
55 ; 因为数据段和代码段重叠，而前面一部分已经被代码段覆盖，所以
56 ; 数据段str_welcome的地址不是 0，而是代码段地址结尾处。
57 ; 堆栈段包含在数据段中，即16MB的末尾部分。
58 ; 实际分布：代码段->数据段->堆栈段
59
60 str_welcome: db "Hello, OS! Welcome to protected mode!",0
61
62 align 2
63 gdt: dw 0,0,0,0 ; 256*4*2bytes = 2KB
64 dw 0x1000,0x0000,0x9a00,0x00c0 ; 16MB
65 dw 0x1000,0x0000,0x9200,0x00c0
66 dw 0x0002,0x8000,0x920b,0x00c0
67 times (256-4) dw 0,0,0,0
68 gdt_end:
69
70 idt: times 256 dw 0,0,0,0 ; 256 interrupts
71 idt_end:
72
73 gdtr: dw 0,0,0
74 align 2
75 idtr: dw 0,0,0
76
77 ;---------------------------------------------------------------------
78 ; stack segment
79
80 stack_buttom: times 512 dw 0 ;1KB
81 stack_top:
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