mit6.828 Lab1

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最新推荐文章于 2023-09-06 10:27:57 发布 · 993 阅读

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本文详细介绍了MIT 6.828实验课程的第一个实验室，涵盖x86汇编语言、GDB调试、JOS操作系统的调试技巧。内容包括使用nasm与GNU汇编器的区别、通过gdb进行调试、32位代码执行的起点及转换原因、boot loader加载kernel的过程、C语言细节、ELF文件结构以及堆栈初始化等知识点。

Part 1

Exercise 1

熟悉x86汇编语言
参考资料：https://pdos.csail.mit.edu/6.828/2018/readings/pcasm-book.pdf
这本书中介绍的是使用nasm汇编器所支持的汇编(Intel Syntax)，但在这个lab中实际使用的是GNU汇编器(AT&T Syntax)
http://www.delorie.com/djgpp/doc/brennan/brennan_att_inline_djgpp.html

Exercise 2

使用gdb对JOS进行调试，调试步骤先在lab的终端中输入make qemu-gdb，再开另外一个终端输入make gdb
用GDB的si指令去追踪BIOS中所用到的指令

Part 2

Exercise 3

查看lab tools guide，这里面包括一些调试OS特殊的GDB技巧

See the GDB manual for a full guide to GDB commands. Here are some particularly useful commands for 6.828, some of which don't typically come up outside of OS development.

Ctrl-c
Halt the machine and break in to GDB at the current instruction. If QEMU has multiple virtual CPUs, this halts all of them.
c (or continue)
Continue execution until the next breakpoint or Ctrl-c.
si (or stepi)
Execute one machine instruction.
b function or b file:line (or breakpoint)
Set a breakpoint at the given function or line.
b *addr (or breakpoint)
Set a breakpoint at the EIP addr.
set print pretty
Enable pretty-printing of arrays and structs.
info registers
Print the general purpose registers, eip, eflags, and the segment selectors. For a much more thorough dump of the machine register state, see QEMU's own info registers command.
x/Nx addr
Display a hex dump of N words starting at virtual address addr. If N is omitted, it defaults to 1. addr can be any expression.
x/Ni addr
Display the N assembly instructions starting at addr. Using $eip as addr will display the instructions at the current instruction pointer.
symbol-file file
(Lab 3+) Switch to symbol file file. When GDB attaches to QEMU, it has no notion of the process boundaries within the virtual machine, so we have to tell it which symbols to use. By default, we configure GDB to use the kernel symbol file, obj/kern/kernel. If the machine is running user code, say hello.c, you can switch to the hello symbol file using symbol-file obj/user/hello.
QEMU represents each virtual CPU as a thread in GDB, so you can use all of GDB's thread-related commands to view or manipulate QEMU's virtual CPUs.

thread n
GDB focuses on one thread (i.e., CPU) at a time. This command switches that focus to thread n, numbered from zero.
info threads
List all threads (i.e., CPUs), including their state (active or halted) and what function they're in.

通过指令b *0x7c00在地址0x7c00中打个断点
通过指令c来时程序运行到断点处
使用si进行单步执行，与boot.S中的指令进行比较
使用x/8i $eip指令来查看从当前执行到的地址开始的8条指令

对boot/main.c中的bootmain()打断点（尚未完成，打不到该函数的断点）

回答以下问题：

在哪里开始处理器开始执行32位代码？是什么导致从16位到32位的转换？
从下面这段开始处理器开始执行32位代码

  # Jump to next instruction, but in 32-bit code segment.
  # Switches processor into 32-bit mode.
  ljmp    $PROT_MODE_CSEG, $protcseg

boot loader的最后一条指令是什么？加载kernel后的第一条指令是什么？
kernel的第一条指令的地址是什么？
为了将整个kernel加载从磁盘加载到内存boot loader如何决定要加载多少个扇区？boot loader从哪里找到要加载的扇区的信息

Exercise 4

推荐弄懂其中的每一个细节，保证对C语言的掌握足以应对接下来的实验
pointer.c

#include <stdio.h>
#include <stdlib.h>

void
f(void)
{
    int a[4];
    int *b = malloc(16);
    int *c;
    int i;

    printf("1: a = %p, b = %p, c = %p\n", a, b, c);

    c = a;
    for (i = 0; i < 4; i++)
	a[i] = 100 + i;
    c[0] = 200;
    printf("2: a[0] = %d, a[1] = %d, a[2] =