! boot.s
!
! It then loads the system at 0x10000, using BIOS interrupts. Thereafter
! it disables all interrupts, changes to protected mode, and calls the
BOOTSEG = 0x07c0
SYSSEG = 0x1000 ! system loaded at 0x10000 (65536).
SYSLEN = 17 ! sectors occupied.
entry start
start:
jmpi go,#BOOTSEG
go: mov ax,cs
mov ds,ax
mov ss,ax
mov sp,#0x400 ! arbitrary value >>512
! ok, we've written the message, now
load_system:
mov dx,#0x0000
mov cx,#0x0002
mov ax,#SYSSEG
mov es,ax
xor bx,bx
mov ax,#0x200+SYSLEN
int 0x13
jnc ok_load
die: jmp die
! now we want to move to protected mode ...
ok_load:
cli ! no interrupts allowed !
mov ax, #SYSSEG
mov ds, ax
xor ax, ax
mov es, ax
mov cx, #0x2000
sub si,si
sub di,di
rep
movw
mov ax, #BOOTSEG
mov ds, ax
lidt idt_48 ! load idt with 0,0
lgdt gdt_48 ! load gdt with whatever appropriate
! absolute address 0x00000, in 32-bit protected mode.
mov ax,#0x0001 ! protected mode (PE) bit
lmsw ax ! This is it!
jmpi 0,8 ! jmp offset 0 of segment 8 (cs)
gdt: .word 0,0,0,0 ! dummy
.word 0x07FF ! 8Mb - limit=2047 (2048*4096=8Mb)
.word 0x0000 ! base address=0x00000
.word 0x9A00 ! code read/exec 代码段，1001 1010 0000 0000 ，S标志位置1(数据段或代码段)
.word 0x00C0 ! granularity=4096, 386
.word 0x07FF ! 8Mb - limit=2047 (2048*4096=8Mb),粒度为4k
.word 0x0000 ! base address=0x00000
.word 0x9200 ! data read/write 1001 0010 0000 0000，注意每一位是干什么用的，运行在特权级0，S标志位置1(数据段或代码段)，扩展方向
.word 0x00C0 ! granularity=4096, 386
idt_48: .word 0 ! idt limit=0
.word 0,0 ! idt base=0L
gdt_48: .word 0x7ff ! gdt limit=2048, 256 GDT entries
.word 0x7c00+gdt,0 ! gdt base = 07xxx
.org 510
.word 0xAA55
