1 ! boot.s 2 ! 3 ! It then loads the system at 0x10000, using BIOS interrupts. Thereafter 4 ! it disables all interrupts, changes to protected mode, and calls the 5 6 BOOTSEG = 0x07c0 7 SYSSEG = 0x1000 ! system loaded at 0x10000 (65536). 8 SYSLEN = 17 ! sectors occupied. 9 10 entry start 11 start: 12 jmpi go,#BOOTSEG 13 go: mov ax,cs 14 mov ds,ax 15 mov ss,ax 16 mov sp,#0x400 ! arbitrary value >>512 17 18 ! ok, we've written the message, now 19 load_system: 20 mov dx,#0x0000 21 mov cx,#0x0002 22 mov ax,#SYSSEG 23 mov es,ax 24 xor bx,bx 25 mov ax,#0x200+SYSLEN 26 int 0x13 27 jnc ok_load 28 die: jmp die 29 30 ! now we want to move to protected mode ... 31 ok_load: 32 cli ! no interrupts allowed ! 33 mov ax, #SYSSEG 34 mov ds, ax 35 xor ax, ax 36 mov es, ax 37 mov cx, #0x2000 38 sub si,si 39 sub di,di 40 rep 41 movw 42 mov ax, #BOOTSEG 43 mov ds, ax 44 lidt idt_48 ! load idt with 0,0 45 lgdt gdt_48 ! load gdt with whatever appropriate 46 47 ! absolute address 0x00000, in 32-bit protected mode. 48 mov ax,#0x0001 ! protected mode (PE) bit 49 lmsw ax ! This is it! 50 jmpi 0,8 ! jmp offset 0 of segment 8 (cs) 51 52 gdt: .word 0,0,0,0 ! dummy 53 54 .word 0x07FF ! 8Mb - limit=2047 (2048*4096=8Mb) 55 .word 0x0000 ! base address=0x00000 56 .word 0x9A00 ! code read/exec 57 .word 0x00C0 ! granularity=4096, 386 58 59 .word 0x07FF ! 8Mb - limit=2047 (2048*4096=8Mb) 60 .word 0x0000 ! base address=0x00000 61 .word 0x9200 ! data read/write 62 .word 0x00C0 ! granularity=4096, 386 63 64 idt_48: .word 0 ! idt limit=0 65 .word 0,0 ! idt base=0L 66 gdt_48: .word 0x7ff ! gdt limit=2048, 256 GDT entries 67 .word 0x7c00+gdt,0 ! gdt base = 07xxx 68 .org 510 69 .word 0xAA55
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