arch/x86/boot/header.S

/*
*	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
.section ".bstext", "ax"

.global bootsect_start
bootsect_start:

# Normalize the start address
ljmp	$BOOTSEG, $start2

start2:
movw	%cs, %ax
movw	%ax, %ds
movw	%ax, %es
movw	%ax, %ss
xorw	%sp, %sp
sti
cld ＃清方向位，在执行串操作时，使地址按递增的方式变化

movw	$bugger_off_msg, %si

;此循环的作用就是将
msg_loop:
lodsb  ＃取一字节到%al
andb	%al, %al
jz	bs_die
movb	$0xe, %ah ＃执行显示服务中断(INT 10H)的第14号功能：在Teletype模式下显示字符
movw	$7, %bx ＃设置前景色(即字符颜色)：白色
int	$0x10
jmp	msg_loop

bs_die:
# Allow the user to press a key, then reboot
xorw	%ax, %ax ＃置%ax的值为0
int	$0x16    ＃调用键盘服务(INT 16H)的00H号功能：从键盘读入字符
int	$0x19    ＃调用系统服务的19H号中断：重启系统

# int 0x19 should never return.  In case it does anyway,
# invoke the BIOS reset code...
ljmp	$0xf000,$0xfff0

.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

# Kernel attributes; used by setup.  This is part 1 of the
# header, from the old boot sector.

.section ".header", "a"
.globl	hdr                     /*传递给内核的部分setup_header的字段*/
hdr:
setup_sects:	.byte 0			/* Filled in by build.c 该扇区之后的实模式代码占用几个扇区*/
root_flags:	.word ROOT_RDONLY
syssize:	.long 0			/* Filled in by build.c */
ram_size:	.word 0			/* Obsolete */
vid_mode:	.word SVGA_MODE
root_dev:	.word 0			/* Filled in by build.c */
boot_flag:	.word 0xAA55

# offset 512, entry point  从偏移地址为512处作为程序的入口点

.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
.byte	start_of_setup-1f
1:

# Part 2 of the header, from the old setup.S

.ascii	"HdrS"		# header signature
.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
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

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 #####################################################

.section ".entrytext", "ax"
start_of_setup:
#ifdef SAFE_RESET_DISK_CONTROLLER
# Reset the disk controller. 磁盘复位
movw	$0x0000, %ax		# Reset disk controller
movb	$0x80, %dl		# All disks
int	$0x13
#endif

# Force %es = %ds
movw	%ds, %ax
movw	%ax, %es
cld

# 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?
movw	%sp, %dx
je	2f		# -> assume %sp is reasonably set

# Invalid %ss, make up a new stack
movw	$_end, %dx
testb	$CAN_USE_HEAP, loadflags
jz	1f
movw	heap_end_ptr, %dx
1:	addw	$STACK_SIZE, %dx
jnc	2f
xorw	%dx, %dx	# Prevent wraparound

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
movzwl	%dx, %esp	# Clear upper half of %esp
sti			# Now we should have a working stack

# We will have entered with %cs = %ds+0x20, normalize %cs so
# it is on par with the other segments.
pushw	%ds  ＃使得%cs = %ds, 消除重定位时可能出现的问题
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"