OS版hello world
其实就是让BIOS拉起一个裸机汇编(实模式),对应是保护模式。
ds段寄存器。
写一个汇编程序helloOs.S
# 以下是AT&T的语法, $开头表示立即数(如自定义符号代表的偏移量)%开头表示寄存器
BOOTSEG = 0x7c0 # 0x7c00
.code16 #告诉汇编编译器as把当前汇编翻译成16位的指令,若不指定会生成x86的32位/64位的指令,因为编译器是64位的
.text
.global _start
_start:
jmpl $BOOTSEG, $start2 #长跳转到start2,会使得段寄存器cs=0x7c0,pc寄存器=start2地址,跳转的目的地是0x7c0+start2地址(偏移量)
# 最终跳转到标签start2偏移量的汇编代码中
start2:
movw $BOOTSEG, %ax # 让ax寄存器=0x7c0
movw %ax, %ds # 进一步初始化其他寄存器
movw %ax, %es
movw %ax, %fs
movw %ax, %gs
movw $msg, %ax #%msg表示msg符号的地址,即把地址放到%ax寄存器里边
movw %ax, %bp
movw $0x01301, %ax #用来指定打印
movw $0x0c, %bx # 设置屏幕显示helloworld的颜色,bh=0(背景色),bl=0c(前景色(即字体)红色)
# movw $0xcf, %bx # 也可换成这行,c会设置成闪烁红色背景色,f会让字体为白色
movw $12, %cx # cx=字符串长度为立即数12
movb $0, %dl
int $0x010 # int中断指令,10h号中断是bios中断,作用就是发起10h号中断,让里边固有函数去cx里读入字符串,让显卡刷新内容
loop:
jmp loop # 本主程序自己进入死循环
msg:
.ascii "hello world"
.org 510 # .org作用是让接下来的510个字节填入0,也就是让汇编编出来的二进制在汇编代码结束后的所有字节填0,目的是为了让编出来的目标文件刚好512字节,刚好一个扇区,因为bios一次会读入一个扇区共512字节到0x7c00的位置。
boot_flag: #在编译出来的二进制文件的最后一个字节设为0xaa55,是bios规定的结束符号。
.word 0xaa55
编译的过程
as helloOs.S -o helloOs.o #as是gcc的汇编编译器,这样会产生一个目标文件helloOs.o
ld -m elf_x86_64 -Ttext 0x0 -s --oformat binary -o helloOs.img helloOs.o #ld是链接器
$ ls -l
-rwxrwxr-x 1 512 5月 13 18:19 helloOs.img
默认情况下,使用as helloOs.S -o helloOs.o编译出来的 helloOs.o是64位的ELF文件,使用readelf能输出如下内容,所有的段都有。
$ readelf -a helloOs.o
ELF 头:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
类别: ELF64
数据: 2 补码,小端序 (little endian)
版本: 1 (current)
OS/ABI: UNIX - System V
ABI 版本: 0
类型: REL (可重定位文件)
系统架构: Advanced Micro Devices X86-64
版本: 0x1
入口点地址: 0x0
程序头起点: 0 (bytes into file)
Start of section headers: 968 (bytes into file)
标志: 0x0
本头的大小: 64 (字节)
程序头大小: 0 (字节)
Number of program headers: 0
节头大小: 64 (字节)
节头数量: 8
字符串表索引节头: 7
节头:
[号] 名称 类型 地址 偏移量
大小 全体大小 旗标 链接 信息 对齐
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000200 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 00000360 #重定位段
0000000000000030 0000000000000018 I 5 1 8
[ 3] .data PROGBITS 0000000000000000 00000240
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 00000240
0000000000000000 0000000000000000 WA 0 0 1
[ 5] .symtab SYMTAB 0000000000000000 00000240
00000000000000f0 0000000000000018 6 9 8
[ 6] .strtab STRTAB 0000000000000000 00000330
000000000000002a 0000000000000000 0 0 1
[ 7] .shstrtab STRTAB 0000000000000000 00000390
0000000000000031 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
There are no section groups in this file.
本文件中没有程序头。
There is no dynamic section in this file.
重定位节 '.rela.text' at offset 0x360 contains 2 entries:
偏移量 信息 类型 符号值 符号名称 + 加数
000000000002 00010000000a R_X86_64_32 0000000000000000 .text + 8
000000000014 00010000000c R_X86_64_16 0000000000000000 .text + 27
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.symtab' contains 10 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 SECTION LOCAL DEFAULT 1
2: 0000000000000000 0 SECTION LOCAL DEFAULT 3
3: 0000000000000000 0 SECTION LOCAL DEFAULT 4
4: 00000000000007c0 0 NOTYPE LOCAL DEFAULT ABS BOOTSEG
5: 0000000000000008 0 NOTYPE LOCAL DEFAULT 1 start2
6: 0000000000000027 0 NOTYPE LOCAL DEFAULT 1 msg
7: 0000000000000025 0 NOTYPE LOCAL DEFAULT 1 loop
8: 00000000000001fe 0 NOTYPE LOCAL DEFAULT 1 boot_flag
9: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 1 _start
No version information found in this file.
如果用objdump -d解析,会输出如下内容
$ objdump -d helloOs.o
helloOs.o: 文件格式 elf64-x86-64
Disassembly of section .text:
0000000000000000 <_start>:
0: 66 ea data16 (bad)
2: 00 00 add %al,(%rax)
4: 00 00 add %al,(%rax)
6: c0 .byte 0xc0
7: 07 (bad)
0000000000000008 <start2>: #能看出与汇编对应
8: b8 c0 07 8e d8 mov $0xd88e07c0,%eax
d: 8e c0 mov %eax,%es
f: 8e e0 mov %eax,%fs
11: 8e e8 mov %eax,%gs
13: b8 00 00 89 c5 mov $0xc5890000,%eax
18: b8 01 13 bb 0c mov $0xcbb1301,%eax
1d: 00 b9 0c 00 b2 00 add %bh,0xb2000c(%rcx)
23: cd 10 int $0x10
0000000000000025 <loop>:
25: eb fe jmp 25 <loop>
0000000000000027 <msg>:
27: 68 65 6c 6c 6f pushq $0x6f6c6c65
2c: 20 77 6f and %dh,0x6f(%rdi)
2f: 72 6c jb 9d <msg+0x76>
31: 64 00 00 add %al,%fs:(%rax)
...
00000000000001fe <boot_flag>:
1fe: 55 push %rbp
1ff: aa stos %al,%es:(%rdi)
如果把这个ELF格式的二进制直接让BIOS读入,显然是无法启动的,毕竟当前电脑上什么都没有,不会识别ELF格式的文件,只有一个固定的BIOS从固定的地址0x7c00处读入内容,所以OS需要的是一个真正的裸机代码开始,通过特定的汇编指令让CPU一步步进入到设定好的流程,直到运行起来复杂的操作系统。
所以我们本次需要编译出来的二进制不能是ELF格式的,不能有重定位段、数据段这些内容,这就需要用链接器来完成。
解释ld命令
ld -m elf_x86_64 -Ttext 0x0 -s --oformat binary -o helloOs.img helloOs.o
# -m elf_x86_64告诉链接器,输入是x86的64位的elf格式的文件helloOs.o
# -Ttext 0x0:-T的是表示代码段,连起来意思是代码段名字为text在二进制文件0x0的位置,即第一个字节开始就是第一条指令
# -s表示把符号去了
# --oformat binary表示让链接器不要翻译bss、重定位等段,仅需翻译代码段即可
最后得到helloOs.img,就是大家所说的镜像文件,再使用objdump发现已不认识该文件了,因为不是elf格式的了
$ objdump -d helloOs.img
objdump: helloOs.img: 不可识别的文件格式
我们可用xxd命令看下该文件的二进制情况,可见熟悉的hello world字符串。
$ xxd -a -u -s 0 -l 512 helloOs.img
00000000: 66EA 0800 0000 C007 B8C0 078E D88E C08E f............... # 66EA是跳转指令
00000010: E08E E8B8 2700 89C5 B801 13BB 0C00 B90C ....'...........
00000020: 00B2 00CD 10EB FE68 656C 6C6F 2077 6F72 .......hello wor
00000030: 6C64 0000 0000 0000 0000 0000 0000 0000 ld..............
00000040: 0000 0000 0000 0000 0000 0000 0000 0000 ................
*
000001f0: 0000 0000 0000 0000 0000 0000 0000 55AA ..............U. # 结束符55AA,在510和512的位置,共512字节
dos系统就是在16位实模式下的一个操作系统。
放入仿真器运行:
硬件模拟器(用于模拟硬件平台,启动OS)http://www.oldlinux.org/Linux.old/bochs/Bochs/bochs-2.6.1/Bochs-2.6.1.exe。安装好后,配置boch的配置文件bochsrc.bxrc,helloOS放在跟它同目录下即可,配置floppya:1_44=helloOs.img, status=inserted 。
#cat bochsrc.bxrc以下内容:
# configuration file generated by Bochs
plugin_ctrl: unmapped=1, biosdev=1, speaker=1, extfpuirq=1, parallel=1, serial=1, gameport=1
config_interface: win32config
display_library: win32
memory: host=32, guest=32 #指定内存32M
romimage: file="D:\Program Files\Bochs-2.6.1/BIOS-bochs-latest" #ROM的模拟文件
vgaromimage: file="D:\Program Files\Bochs-2.6.1/VGABIOS-lgpl-latest" #显卡的模拟文件
boot: floppy #指定从软盘启动,写成boot:a也可,表示从软盘a启动,因为a和b盘都是软盘。c是硬盘,但从硬盘启动更复杂,需要很多硬盘知识,后期再介绍。
floppy_bootsig_check: disabled=0
# no floppya
floppya:1_44=helloOs.img, status=inserted #定义软盘和装载的内容,就是写的helloworld版os:helloOs.img
# no floppyb
ata0: enabled=1, ioaddr1=0x1f0, ioaddr2=0x3f0, irq=14
ata0-master: type=none
ata0-slave: type=none
ata1: enabled=1, ioaddr1=0x170, ioaddr2=0x370, irq=15
ata1-master: type=none
ata1-slave: type=none
ata2: enabled=0
ata3: enabled=0
pci: enabled=1, chipset=i440fx
vga: extension=vbe, update_freq=5
cpu: count=1, ips=4000000, model=bx_generic, reset_on_triple_fault=1, cpuid_limit_winnt=0, ignore_bad_msrs=1, mwait_is_nop=0
cpuid: vendor_string="GenuineIntel", brand_string=" Intel(R) Pentium(R) 4 CPU "
cpuid: stepping=3, model=3, family=6, mmx=1, apic=xapic, sse=sse2, sse4a=0, misaligned_sse=0
cpuid: sep=1, movbe=0, adx=0, aes=0, xsave=0, xsaveopt=0, x86_64=1, 1g_pages=0, pcid=0
cpuid: fsgsbase=0, smep=0, smap=0, mwait=1
print_timestamps: enabled=0
port_e9_hack: enabled=0
private_colormap: enabled=0
clock: sync=none, time0=local, rtc_sync=0
# no cmosimage
# no loader
log: -
logprefix: %t%e%d
debug: action=ignore
info: action=report
error: action=report
panic: action=ask
keyboard: type=mf, serial_delay=250, paste_delay=100000, user_shortcut=none
mouse: type=ps2, enabled=0, toggle=ctrl+mbutton
parport1: enabled=1, file=none
parport2: enabled=0
com1: enabled=1, mode=null, dev=none
com2: enabled=0
com3: enabled=0
com4: enabled=0
启动boch后,在弹出窗里选Load,选中bochsrc.bxrc
可见启动成功。在安装目录下还有个bochsdbg.exe,可用bochsdbg.exe -q -f bochsrc.bxrc就可启动调试模式,单步调试刚刚的汇编程序了
也可用图形界面启动dbg模式,一样是load配置文件,然后进入如下界面
Next at t=0
(0) [0x0000fffffff0] f000:fff0 (unk. ctxt): jmp far f000:e05b ; ea5be000f0
<bochs:1> n
Next at t=1
(0) [0x0000000fe05b] f000:e05b (unk. ctxt): xor ax, ax ; 31c0
<bochs:2> n
00000000002i[WGUI ] dimension update x=720 y=400 fontheight=16 fontwidth=9 bpp=8
Next at t=2
(0) [0x0000000fe05d] f000:e05d (unk. ctxt): out 0x0d, al ; e60d
<bochs:3> b 0x7c00
<bochs:4> c
00000000025i[MEM0 ] allocate_block: block=0x0 used 0x1 of 0x20
00000004661i[BIOS ] $Revision: 11545 $ $Date: 2012-11-11 09:11:17 +0100 (So, 11. Nov 2012) $
00000318067i[KBD ] reset-disable command received
00000320801i[BIOS ] Starting rombios32
00000321235i[BIOS ] Shutdown flag 0
00000321830i[BIOS ] ram_size=0x02000000
00000322251i[BIOS ] ram_end=32MB
00000362768i[BIOS ] Found 1 cpu(s)
00000376955i[BIOS ] bios_table_addr: 0x000fa448 end=0x000fcc00
00000704752i[PCI ] i440FX PMC write to PAM register 59 (TLB Flush)
00001032679i[P2I ] PCI IRQ routing: PIRQA# set to 0x0b
00001032698i[P2I ] PCI IRQ routing: PIRQB# set to 0x09
00001032717i[P2I ] PCI IRQ routing: PIRQC# set to 0x0b
00001032736i[P2I ] PCI IRQ routing: PIRQD# set to 0x09
00001032746i[P2I ] write: ELCR2 = 0x0a
00001033512i[BIOS ] PIIX3/PIIX4 init: elcr=00 0a
00001041228i[BIOS ] PCI: bus=0 devfn=0x00: vendor_id=0x8086 device_id=0x1237 class=0x0600
00001043498i[BIOS ] PCI: bus=0 devfn=0x08: vendor_id=0x8086 device_id=0x7000 class=0x0601
00001045607i[BIOS ] PCI: bus=0 devfn=0x09: vendor_id=0x8086 device_id=0x7010 class=0x0101
00001045836i[PIDE ] new BM-DMA address: 0xc000
00001046453i[BIOS ] region 4: 0x0000c000
00001048455i[BIOS ] PCI: bus=0 devfn=0x0a: vendor_id=0x8086 device_id=0x7020 class=0x0c03
00001048659i[UHCI ] new base address: 0xc020
00001049276i[BIOS ] region 4: 0x0000c020
00001049402i[UHCI ] new irq line = 9
00001051287i[BIOS ] PCI: bus=0 devfn=0x0b: vendor_id=0x8086 device_id=0x7113 class=0x0680
00001051520i[ACPI ] new irq line = 11
00001051532i[ACPI ] new irq line = 9
00001051561i[ACPI ] new PM base address: 0xb000
00001051575i[ACPI ] new SM base address: 0xb100
00001051603i[PCI ] setting SMRAM control register to 0x4a
00001215694i[CPU0 ] Enter to System Management Mode
00001215704i[CPU0 ] RSM: Resuming from System Management Mode
00001379722i[PCI ] setting SMRAM control register to 0x0a
00001394656i[BIOS ] MP table addr=0x000fa520 MPC table addr=0x000fa450 size=0xc8
00001396412i[BIOS ] SMBIOS table addr=0x000fa530
00001396470i[MEM0 ] allocate_block: block=0x1f used 0x2 of 0x20
00001398607i[BIOS ] ACPI tables: RSDP addr=0x000fa650 ACPI DATA addr=0x01ff0000 size=0xf72
00001401804i[BIOS ] Firmware waking vector 0x1ff00cc
00001403602i[PCI ] i440FX PMC write to PAM register 59 (TLB Flush)
00001404330i[BIOS ] bios_table_cur_addr: 0x000fa674
00001531947i[VBIOS] VGABios $Id: vgabios.c,v 1.75 2011/10/15 14:07:21 vruppert Exp $
00001532018i[BXVGA] VBE known Display Interface b0c0
00001532050i[BXVGA] VBE known Display Interface b0c5
00001534975i[VBIOS] VBE Bios $Id: vbe.c,v 1.64 2011/07/19 18:25:05 vruppert Exp $
00014040328i[BIOS ] Booting from 0000:7c00
(0) Breakpoint 1, 0x0000000000007c00 in ?? ()
Next at t=14040383
(0) [0x000000007c00] 0000:7c00 (unk. ctxt): jmp far 07c0:00000008 ; 66ea08000000c007
<bochs:5> n
Next at t=14040384
(0) [0x000000007c08] 07c0:0008 (unk. ctxt): mov ax, 0x07c0 ; b8c007
<bochs:6> sreg
es:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
cs:0x07c0, dh=0x00009300, dl=0x7c00ffff, valid=1
Data segment, base=0x00007c00, limit=0x0000ffff, Read/Write, Accessed
ss:0x0000, dh=0x00009300, dl=0x0000ffff, valid=7
Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ds:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
fs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
gs:0x0000, dh=0x00009300, dl=0x0000ffff, valid=1
Data segment, base=0x00000000, limit=0x0000ffff, Read/Write, Accessed
ldtr:0x0000, dh=0x00008200, dl=0x0000ffff, valid=1
tr:0x0000, dh=0x00008b00, dl=0x0000ffff, valid=1
gdtr:base=0x00000000000fa1b7, limit=0x30
idtr:base=0x0000000000000000, limit=0x3ff
<bochs:7> n
Next at t=14040385
(0) [0x000000007c0b] 07c0:000b (unk. ctxt): mov ds, ax ; 8ed8
<bochs:8> n
Next at t=14040386
(0) [0x000000007c0d] 07c0:000d (unk. ctxt): mov es, ax ; 8ec0
<bochs:9>
Next at t=14040387
(0) [0x000000007c0f] 07c0:000f (unk. ctxt): mov fs, ax ; 8ee0
<bochs:10>
Next at t=14040388
(0) [0x000000007c11] 07c0:0011 (unk. ctxt): mov gs, ax ; 8ee8
<bochs:11>
Next at t=14040389
(0) [0x000000007c13] 07c0:0013 (unk. ctxt): mov ax, 0x0027 ; b82700
<bochs:12> x/20xh 0x7c27 # 这个就是hello world
[bochs]:
0x0000000000007c27 <bogus+ 0>: 0x6568 0x6c6c 0x206f 0x6f77 0x6c72 0x0064 0x0000 0x0000
第一个程序用的是AT&T的语法,与intel语法相比,简单区别
AT&T语法:立即数用%号开头,寄存器用%开头,movw %ax, %ds表示%ax(源操作数)的内容复制到%ds(目的操作数)。
intel语法:mov 目的操作数,源操作数
扩展:
标签:helloOs,00,0000,0000000000000000,BIOS,ax,world,OS,hello From: https://www.cnblogs.com/UFO-blogs/p/17397994.html汇编的后缀名一般是.s和.S
小写.s:表示改汇编程序只含汇编代码,编译器不会进行预编译
大写.S:表示含预编译代码,希望编译器进行预编译(宏替换)