xv6的引导程序
当x86的PC启动的时候,它首先执行的程序是BIOS,该段程序存储在主板的flash内存中,
BIOS的任务有以下两种:
1.为后面程序的运行初始化硬件
2.把控制权转移到操作系统,尤其是转移到从boot sector中读取的代码处,该boot sector是boot disk的第一个512字节的sector,
BIOS加载扇区的内容到内存的0x7c00处,然后跳转到该地址(设置处理的ip值)处。
当boot sector开始执行的时候,处理器是运行在Intel 的8088的模式下(实模式),
xv6的boot sector的任务如下:
1.让处理器运行到更加现代的操作模式下,
2.从磁盘上加载xv6的内核,然后转移控制权到内核处。
在xv6中,boot sector有两个源代码文件组成,一个是用汇编语言编写的(16-bit和32-bit的x86汇编)代码bootasm.S,另一是用C语言编写的代码bootmain.c
下面介绍boot sector的操作过程,从BIOS执行开始,直到转移控制权到内核处。
boot sector可以看着是内核自身的一个缩影,它包含低级的汇编代码和C代码,它管理着自己的内存,它甚至含有一个设备驱动程序,而所有的这一切都在512字节的机器代码中。
以下分别对bootasm.S和bootmain.c文件中的代码进行分析
1.bootasm.S中的代码如下(主要完成处理器从实模式转换到32位的保护模式):
1 #include "asm.h"
2
3 # Start the first CPU: switch to 32-bit protected mode, jump into C.
4 # The BIOS loads this code from the first sector of the hard disk into
5 # memory at physical address 0x7c00 and starts executing in real mode
6 # with %cs=0 %ip=7c00.
7
8 #define SEG_KCODE 1 // kernel code
9 #define SEG_KDATA 2 // kernel data+stack
10
11 #define CR0_PE 1 // protected mode enable bit
12
13 .code16 # Assemble for 16-bit mode
14 .globl start
15 start:
16 cli # Disable interrupts
17
18 # Set up the important data segment registers (DS, ES, SS).
19 xorw %ax,%ax # Segment number zero
20 movw %ax,%ds # -> Data Segment
21 movw %ax,%es # -> Extra Segment
22 movw %ax,%ss # -> Stack Segment
23
24 # Enable A20:
25 # For backwards compatibility with the earliest PCs, physical
26 # address line 20 is tied low, so that addresses higher than
27 # 1MB wrap around to zero by default. This code undoes this.
28 seta20.1:
29 inb $0x64,%al # Wait for not busy
30 testb $0x2,%al
31 jnz seta20.1
32
33 movb $0xd1,%al # 0xd1 -> port 0x64
34 outb %al,$0x64
35
36 seta20.2:
37 inb $0x64,%al # Wait for not busy
38 testb $0x2,%al
39 jnz seta20.2
40
41 movb $0xdf,%al # 0xdf -> port 0x60
42 outb %al,$0x60
43
44 # Switch from real to protected mode, using a bootstrap GDT
45 # and segment translation that makes virtual addresses
46 # identical to physical addresses, so that the
47 # effective memory map does not change during the switch.
48 lgdt gdtdesc
49 movl %cr0, %eax
50 orl $CR0_PE, %eax
51 movl %eax, %cr0
52
53 # This ljmp is how you load the CS (Code Segment) register.
54 # SEG_ASM produces segment descriptors with the 32-bit mode
55 # flag set (the D flag), so addresses and word operands will
56 # default to 32 bits after this jump.
57 ljmp $(SEG_KCODE<<3), $start32
58
59 .code32 # Assemble for 32-bit mode
60 start32:
61 # Set up the protected-mode data segment registers
62 movw $(SEG_KDATA<<3), %ax # Our data segment selector
63 movw %ax, %ds # -> DS: Data Segment
64 movw %ax, %es # -> ES: Extra Segment
65 movw %ax, %ss # -> SS: Stack Segment
66 movw $0, %ax # Zero segments not ready for use
67 movw %ax, %fs # -> FS
68 movw %ax, %gs # -> GS
69
70 # Set up the stack pointer and call into C.
71 movl $start, %esp
72 call bootmain
73
74 # If bootmain returns (it shouldn't), trigger a Bochs
75 # breakpoint if running under Bochs, then loop.
76 movw $0x8a00, %ax # 0x8a00 -> port 0x8a00
77 movw %ax, %dx
78 outw %ax, %dx
79 movw $0x8ae0, %ax # 0x8ae0 -> port 0x8a00
80 outw %ax, %dx
81 spin:
82 jmp spin
83
84 # Bootstrap GDT
85 .p2align 2 # force 4 byte alignment
86 gdt:
87 SEG_NULLASM # null seg
88 SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
89 SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
90
91 gdtdesc:
92 .word (gdtdesc - gdt - 1) # sizeof(gdt) - 1
93 .long gdt # address gdt
bootsector中第一条指令为cli(1行),用于禁止处理器的中断,中断是一种用于硬件设备调用操作系统中断处理程序的方式,BIOS可以看作一个小型的操作系统,因此其有它自己的中断处理程序,作为硬件初始化的一部分,但是boot sector运行的时候,BIOS已不再运行了,且此时不适合处理硬件设备的中断。当xv6内核准备就绪后,将会重新启动中断。由于当BIOS运行完后,我们并不知道ds,es,ss里存储的是什么内容,因此,我们需要做的是设置寄存器中的值为0,然后分别把ax中的值copy到ds,es,ss(19~22行)。
lgdt gdtdesc指令(48行):将GDT表的首地址加载到GDTR中,(49~51行):将cr0寄存器的最低位设置为1,标志着系统进入保护模式,
ljmp指令(57行):让系统开始使用23位的寻址模式,该指令是在系统进入保护模式后执行的,因此,$(SEG_KCODE << 3)会被存入寄存器cs中,代表的是段选择子,从GDT表的定义可以看到基地址为0x0,而偏移地址为:$start32,$start32实际上表示的是接下来指令的链接地址,也就是可执行程序在内存中的虚拟地址,只是刚好在这里编译生成的可执行程序boot的加载地址与链接地址是一致的。因此,可以跳转成功。关于链接地址与加载地址可参考邵志远老师的32位操作系统实践课程
进入保护模式后,程序就重新对段寄存器进行了初始化,并且对堆栈指针进行了赋值,然后便调用bootmain函数。下面对bootmain.c进行分析
2.bootmain.c源代码如下(完成将内核的可执行代码从硬盘中读入到内存):
// Boot loader.
//
// Part of the boot sector, along with bootasm.S, which calls bootmain().
// bootasm.S has put the processor into protected 32-bit mode.
// bootmain() loads an ELF kernel image from the disk starting at
// sector 1 and then jumps to the kernel entry routine.
#include "types.h"
#include "elf.h"
#include "x86.h"
#define SECTSIZE 512
void readseg(uchar*, uint, uint);
void
bootmain(void)
{
struct elfhdr *elf;
struct proghdr *ph, *eph;
void (*entry)(void);
uchar* va;
elf = (struct elfhdr*)0x10000; // scratch space
// Read 1st page off disk
readseg((uchar*)elf, 4096, 0);
// Is this an ELF executable?
if(elf->magic != ELF_MAGIC)
return; // let bootasm.S handle error
// Load each program segment (ignores ph flags).
ph = (struct proghdr*)((uchar*)elf + elf->phoff);
eph = ph + elf->phnum;
for(; ph < eph; ph++) {
va = (uchar*)(ph->va & 0xFFFFFF);
readseg(va, ph->filesz, ph->offset);
if(ph->memsz > ph->filesz)
stosb(va + ph->filesz, 0, ph->memsz - ph->filesz);
}
// Call the entry point from the ELF header.
// Does not return!
entry = (void(*)(void))(elf->entry & 0xFFFFFF);
entry();
}
void
waitdisk(void)
{
// Wait for disk ready.
while((inb(0x1F7) & 0xC0) != 0x40)
;
}
// Read a single sector at offset into dst.
void
readsect(void *dst, uint offset)
{
// Issue command.
waitdisk();
outb(0x1F2, 1); // count = 1
outb(0x1F3, offset);
outb(0x1F4, offset >> 8);
outb(0x1F5, offset >> 16);
outb(0x1F6, (offset >> 24) | 0xE0);
outb(0x1F7, 0x20); // cmd 0x20 - read sectors
// Read data.
waitdisk();
insl(0x1F0, dst, SECTSIZE/4);
}
// Read 'count' bytes at 'offset' from kernel into virtual address 'va'.
// Might copy more than asked.
void
readseg(uchar* va, uint count, uint offset)
{
uchar* eva;
eva = va + count;
// Round down to sector boundary.
va -= offset % SECTSIZE;
// Translate from bytes to sectors; kernel starts at sector 1.
offset = (offset / SECTSIZE) + 1;
// If this is too slow, we could read lots of sectors at a time.
// We'd write more to memory than asked, but it doesn't matter --
// we load in increasing order.
for(; va < eva; va += SECTSIZE, offset++)
readsect(va, offset);
}
待续。。。。