从开始到 # offset 512, entry point 功能和以前的bootsect.
后面的功能和setup.S的一部分
类似：
1：设置setup header参数部分
2：start_of_setup
1）：设置堆栈
2）：检查setup中的标签
3）：清除BSS段
4）：调用C入口main
/*
* header.S
*
* Copyright (C) 1991, 1992
Linus Torvalds
*
* Based on bootsect.S and setup.S
* modified
by more people than can be counted
*
* Rewritten as a common
file by H. Peter Anvin (Apr 2007)
*
* BIG FAT NOTE: We're in
real mode using 64k segments. Therefore segment
* addresses must be
multiplied by 16 to obtain their respective linear
* addresses. To
avoid confusion, linear addresses are written using leading
* hex
while segment addresses are written as segment:offset.
*
*/
#include <asm/segment.h>
#include
<linux/utsrelease.h>
#include <asm/boot.h>
#include
<asm/e820.h>
#include <asm/page_types.h>
#include
<asm/setup.h>
#include "boot.h"
#include "voffset.h"
#include
"zoffset.h"
BOOTSEG = 0x07C0 /* original
address of boot-sector */
SYSSEG = 0x1000 /* historical load
address >> 4 */
#ifndef SVGA_MODE
#define SVGA_MODE ASK_VGA
#endif
#ifndef RAMDISK
#define RAMDISK 0
#endif
#ifndef ROOT_RDONLY
#define ROOT_RDONLY 1
#endif
.code16 //以下为16位模式
代码
.section ".bstext", "ax" //定义一个
。bstext段，这个段是可写'a'和可执行'x'的
.global bootsect_start
//AT&T汇编语法中, .global 修饰使得 符号bootsect_start 对ld可见
bootsect_start:
//符号定义
# Normalize the start address
ljmp $BOOTSEG,
$start2 //跳转到 07C0的偏移 start2处
start2:
movw %cs, %ax
movw %ax, %ds
movw %ax,
%es
movw %ax, %ss //将ds,es,ss全设置为cs
xorw %sp,
%sp //将sp置0
sti //开中断
cld //清方向标志
movw $bugger_off_msg, %si //把下
面bugger_off_msg 符号的偏移地址放在 si寄存器中
msg_loop:
lodsb
//把si指向的源串的内容逐步装入al 中 PS:stosb 指的是将al中数据装入di指向的地址中.
andb %al, %al //当取完的时候,al=0
,此时操作andb后方向为0,执行下句跳转
jz bs_die
movb $0xe, %ah
movw $7,
%bx
int $0x10 // 视频服务中断指令, AH = 0Eh, AL = Character, BL = Color
(only in graphic mode)
jmp msg_loop
bs_die:
# Allow the user to press a key, then
reboot
xorw %ax, %ax
int $0x16
int $0x19
# int 0x19 should never return. In case it does
anyway,
# invoke the BIOS reset code...
ljmp $0xf000,$0xfff0 //跳转到bios入口 fff0
.section ".bsdata", "a"
bugger_off_msg:
.ascii "Direct
booting from floppy is no longer supported./r/n"
.ascii "Please use
a boot loader program instead./r/n"
.ascii "/n"
.ascii "Remove
disk and press any key to reboot . . ./r/n"
.byte 0
#可以看出以上的bootsect无任何实际意义,其实2.6以后的
内核需要另外的bootloader才可以 ,例如grub等.
在grub的boot_func中的big_linux_boot
里，描述了实际上grub的stage2将内核的
bootsect和setup实模式代码载入到地址0x90000后，是skip了头0x200个字节的，直接跳转到地址0x90200处执行的。
# Kernel attributes; used by
setup. This is part 1 of the header, from the old boot sector.
.section ".header", "a"
//header段
.globl hdr
//定义一个全局的符号
hdr:
setup_sects: .byte
0 /* Filled in by build.c */ //.byte是类型定义
,这行及以下是一个首部的设置 01F1
root_flags: .word
ROOT_RDONLY //01F2
syssize: .long
0 /* Filled in by build.c */ //01F4
ram_size: .word
0 /* Obsolete */
vid_mode: .word SVGA_MODE
root_dev: .word
0 /* Filled in by build.c */
boot_flag: .word 0xAA55
//01FE
# offset 512, entry point
以上生成的程序偏
移地址从 0000~~~01ff 就是512Bytes的bootsect.
------------------------------------------------------------------------------------------------------------------------------------------------------------------
下面的就是真正的setup代码了,从偏移0x0200开始.0200
位置处有一个Jump指令
.globl _start
_start:
# Explicitly enter
this as bytes, or the assembler
# tries to generate a 3-byte jump
here, which causes
# everything else to push off to the wrong
offset.
.byte 0xeb # short (2-byte) jump
//0200的第一个字节是跳转指令.
.byte start_of_setup-1f
//0201是跳转距离,(start_of_setup-1f)其实就是 setup中的头部长度.
1:
# Part 2 of the header, from
the old setup.S
.ascii "HdrS" # header
signature //0202
.word 0x020a # header version number (>=
0x0105)
# or else old loadlin-1.5 will fail)
.globl realmode_swtch
realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
start_sys_seg: .word SYSSEG
#
obsolete and meaningless, but just
# in case something decided
to "use" it
.word kernel_version-512 #
pointing to kernel version string
# above section of
header is compatible
# with loadlin-1.5 (header v1.5). Don't
#
change it.
type_of_loader: .byte 0
#
0 means ancient bootloader, newer
# bootloaders know to change
this.
# See Documentation/i386/boot.txt for
# assigned
ids
# flags, unused bits must be zero (RFU) bit within
loadflags
loadflags:
LOADED_HIGH = 1 # If set, the kernel is loaded high
//大内核
CAN_USE_HEAP =
0x80
# If set, the loader also has set
#
heap_end_ptr to tell how much
# space behind setup.S can be used
for
# heap purposes.
# Only the loader knows what is
free
.byte LOADED_HIGH //添加符号
setup_move_size: .word
0x8000 # size to move, when setup is not
# loaded at
0x90000. We will move setup
# to 0x90000 then just before
jumping
# into the kernel. However, only the
# loader
knows how much data behind
# us also needs to be loaded.
code32_start: # here loaders can put a different
#
start address for 32-bit code.
.long 0x100000 # 0x100000 = default
for big kernel
ramdisk_image: .long 0 # address of loaded ramdisk
image
# Here the loader puts the 32-bit
# address where
it loaded the image.
# This only will be read by the kernel.
ramdisk_size: .long 0 # its size in bytes
bootsect_kludge:
.long 0 # obsolete
heap_end_ptr: .word _end+STACK_SIZE-512
#
(Header version 0x0201 or later)
# space from here (exclusive)
down to
# end of setup code can be used by setup
# for
local heap purposes.
ext_loader_ver:
.byte 0 # Extended boot loader
version
ext_loader_type:
.byte 0 # Extended boot loader type
cmd_line_ptr: .long 0 # (Header version 0x0202 or
later)
# If nonzero, a 32-bit pointer
# to the kernel
command line.
# The command line should be
# located
between the start of
# setup and the end of low
# memory
(0xa0000), or it may
# get overwritten before it
# gets
read. If this field is
# used, there is no longer
#
anything magical about the
# 0x90000 segment; the setup
#
can be located anywhere in
# low memory 0x10000 or higher.
ramdisk_max: .long 0x7fffffff
# (Header
version 0x0203 or later)
# The highest safe address for
#
the contents of an initrd
# The current kernel allows up to 4
GB,
# but leave it at 2 GB to avoid
# possible
bootloader bugs.
kernel_alignment: .long
CONFIG_PHYSICAL_ALIGN #physical addr alignment
#required for
protected mode
#kernel
#ifdef CONFIG_RELOCATABLE
relocatable_kernel:
.byte 1
#else
relocatable_kernel: .byte 0
#endif
min_alignment: .byte
MIN_KERNEL_ALIGN_LG2 # minimum alignment
pad3: .word 0
cmdline_size: .long COMMAND_LINE_SIZE-1
#length of the command line,
#added with boot protocol
#version 2.06
hardware_subarch: .long 0 # subarchitecture, added
with 2.07
# default to 0 for normal x86 PC
hardware_subarch_data: .quad 0
payload_offset: .long ZO_input_data
payload_length: .long
ZO_z_input_len
setup_data: .quad 0 # 64-bit physical pointer to
#
single linked list of
# struct setup_data
pref_address: .quad LOAD_PHYSICAL_ADDR # preferred
load addr
#define ZO_INIT_SIZE (ZO__end - ZO_startup_32 +
ZO_z_extract_offset)
#define VO_INIT_SIZE (VO__end - VO__text)
#if
ZO_INIT_SIZE > VO_INIT_SIZE
#define INIT_SIZE ZO_INIT_SIZE
#else
#define
INIT_SIZE VO_INIT_SIZE
#endif
init_size: .long INIT_SIZE #
kernel initialization size
# End of setup
header ,首部设置完毕,现在开始start_of_setup:
.section ".entrytext", "ax" //声明一个段
start_of_setup:
#ifdef SAFE_RESET_DISK_CONTROLLER
#
Reset the disk controller.
movw $0x0000, %ax # Reset disk
controller
movb $0x80, %dl # All disks
int $0x13
#用13号中断重设磁盘控制器 ax=0x0 ，dl=0x80
#endif
# Force %es = %ds #ds：数据段 es：附加数据段
movw %ds,
%ax
movw %ax, %es
cld #清方向标志位DF=0
# Apparently some ancient versions of LILO invoked the
kernel with %ss != %ds,
# which happened to work by accident for the
old code. Recalculate the stack
# pointer if %ss is invalid.
Otherwise leave it alone, LOADLIN sets up the
# stack behind its own
code, so we can't blindly put it directly past the heap.
movw %ss, %dx
cmpw %ax, %dx #
%ds == %ss? #ax里的还是ds的值，现在就是比较 ds和ss的值
movw %sp, %dx #再把dx设置位栈顶指针值
je 2f #
-> assume %sp is reasonably set #如果比较相等，则跳转到 前面（f表前，b表后）的 标号2处
，不相等就新建一个栈
# Invalid %ss, make up a new stack #ds和ss不相等的情况下
movw $_end, %dx #把栈底设置给dx
testb $CAN_USE_HEAP,
loadflags #testb：两操作数（byte）做与运算，只修改标志位. 看内核头参数中有没有设置 CAN_USE_HEAP位.
jz 1f
#如果与运算结果等于0，即设置了这一位则跳到下面标号1处
movw heap_end_ptr, %dx
#否则用heap_end_ptr变量作为栈底
1: addw $STACK_SIZE, %dx
#将栈的大小加上设置的
jnc 2f //无进位时转移到标号2处
xorw %dx,
%dx # Prevent wraparound //,若有进位则将dx设置为0
2:
# Now
%dx should point to the end of our stack space
andw $~3,
%dx # dword align (might as well...)
jnz 3f
movw $0xfffc,
%dx # Make sure we're not zero
3: movw %ax, %ss
//ss=ds了
movzwl %dx, %esp # Clear upper half of %esp //只用低16位的sp
sti # Now we should have a
working stack #sti：置中断允许位
# We will have entered with %cs = %ds+0x20, normalize
%cs so
# it is on par with the other segments.
pushw %ds
pushw $6f
lretw
6:
# Check signature at end of setup
cmpl $0x5a5aaa55, setup_sig
jne setup_bad
# Zero the bss
movw $__bss_start,
%di
movw $_end+3, %cx
xorl %eax, %eax
subw %di, %cx
shrw $2,
%cx
rep; stosl
# Jump to C code (should not return)
calll main
# Setup corrupt somehow...
setup_bad:
movl $setup_corrupt,
%eax
calll puts
# Fall through...
.globl die
.type die, @function
die:
hlt
jmp die
.size die, .-die
.section ".initdata", "a"
setup_corrupt:
.byte 7
.string "No
setup signature found.../n"