技能学习

最近开始学习南京大学计算机系统实验，刚刚完成PA0——开发环境配置。从安装 GNU/Linux 系统，熟悉Linux常用命令，学习Makefile，GDB，Git，TMux工具的的使用，编译并运行NEMU（全系统模拟器）。
PA0实验中遇到印象非常深刻的问题是：

在Installing Tools环节，系统崩溃自动重启无法开机，安装的是双系统，ubuntu 20.04 LTS。第一次系统崩溃不知道是什么问题，以为自己指令操作不当导致。只好重新安装系统（仍然是用之前做好的Ubuntu20.04LTS U盘），安装输入法、翻墙工具然后又是Installing Tools环节，无法关机（右上角的Poweroff按钮消失）后使用终端poweroff关机，此时已经凌晨2点以为解决了。早上起来无法开机（心态有点崩），在怀疑是系统与软件兼容问题？不过不是LTS版本吗？带着疑惑试了第三次重装，果然是一样的结果。后重新下载ubuntu 21.04，后来没有故障。由于自己对linux的不熟悉导致了浪费了很多时间，不过也熟悉了不少。
在Compiling and Running NEMU环节，在执行make menuconfig命令后出现报错，官方给的方案是自己解决。一开始看着报错摸不着头脑，分析报错信息，查看nemu文件构成，功能等，半个小时过去了没解决（就开始作弊了，百度搜索…不过还好也没搜到解决方案），只能接着想后来在报错信息发现了端倪和官方的提示（缺少的工具装上），根据信息装上了两个工具后成功解决这个报错。虽然是个小问题，但无不反应普通高校学校的同学对学术欺骗的态度，和面对问题的独立解决能力差和不耐心，值得自己反思。

这是实验的开端，官方预计解决时间是10h，我花了23h，可见与南大学子的差距。此次实验也是对自己能力的调整与突破：

尝试全英文学习，使用英文doc，全英文版ubuntu，全英文查阅问题
尽可能做到学术诚信，不抄袭，不参考他人代码，独立解决

PA0

First Exploration with GNU/Linux

GUI and CLI

if you stick to GUI, you can only do what it can; but in CLI(Command Line Interface), it can do what you want.

Difference Between “su” and “su -” Commands

Linux User Environment: Linux’s systems are multi-user environments. Whenever Linux operating system creates a new shell session(a new terminal) it started preparing an environment for itself. This environment basically holds the Environment variable(Environment depends on shell type, Bash is generally used by most of the Linux distribution).
For example, pwd command is an environmental variable.

su command is an abbreviation(缩写) for “substitute user” because it is used for switching to another user during a normal login session, but it is often mistaken as an abbreviation for “super user” as mainly su command is used for getting “super user” privileges.

Su Command

$whoami
ryan #show current user$pwd
/home/ryan #show cuurent directory$su root #not create a new User Environment#whoami
root #pwd
/home/ryan #the same

Su - Command

$whoami
ryan #show current user$pwd
/home/ryan #show current directory$su - root #Create a new User Environment#whoami
root#pwd
/root

su command does not create a new User Environment (in the simple term they pretend to be the target user) but su - **creates a totally new **User Environment(in the simple term they are actually the target user).

Sudo Command

sudo command executes instructions as a system administrator, that is to say, the instructions executed via sudo are as if they were executed by root himself.

More Exploration

Man Command

man an interface to the system reference manuals.

Inputman ls, “LS(1)” is displayed in the upper left corner, “LS” means manual’s name and “(1)” means this in the first chapter.

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Makefile

Makefile is a program-building tool that runs on Unix, Linux, and their flavors. It aids in simplifying building program executables that may need various modules. To determine how the modules need to be compiled or recompiled, make takes the help of user-defined makefiles. Makefile guides the make utility while compiling and linking program modules.

Compiling the source code files can be tiring, especially when you have to include several source files and type the compiling command every time you need to compile. Makefiles are the solution to simplify this task.

Makefiles are speical format files that help build and manage the projects automatically.

The make command allows you to manage large programs or groups of programs. As you begin to write large programs, you notice that re-compiling large programs take a longer time than re-compiling short programs. Moreover, you notice that you usually only work on a small section of the program, and much of reaming program is unchanged.

Difference between g++ & gcc


g++	gcc
g++ is used to compile C++ program.	gcc is used to compile C program
g++ can compile any .c or .cpp files but they will be treated as C++ files only.	gcc can compile any .c or .cpp files but they will be treated as C and C++ respectively.
g++ fileName.cpp -o binary	gcc fileName.c -o binary
Using g++ to link the object files, file automatically links in the std C++ libraries.	gcc does not do this.

Defining Rules in Makefile

target [target...] : [dependent...]
[command...]

In the above code, the arguments in brackets are optional and ellipsis means one or more.Here, note that the tab to preface each command is required.

Make How to Execute

Find “Makefile” or “makefile” files in the current directory.
If you find it, make will search the first target of the makefile, such as the main file(executable file).
If the main file is not found or the .o files that the main file depends on are newer than it, execute the command to generate the main file.
If the .o files are not found, make will find the dependent files of the .o files and execute the command to generate .o files(It likes the process of the stack).
If the .o files and .h files are both found, make will generate .o files then use .o files to achieve the final task - - - generate the main file.

Special Macros


$@	The file name of the target.
$<	The name of the first dependency.
$*	The part of filename which matched a suffix rule.
$?	The names of all the dependencies newer than the target separated by spaces.
$^	The names of all the dependencies separated by spaces, but with duplicate names removed.

GDB

GDB, short for GNU Debugger, is the most popular debugger for UNIX systems to debug C and C++ programs. A debugger is a program that runs other programs, allowing the user to exercise control over these programs, and to examine variables when problems arise.

GDB Command

Getting Started: Starting and Stopping

gcc -g main.c - Compiles main.c with the debugging option (-g). You still get an a.out, but it contains debugging information that lets you use variables and function names inside GDB.
gdb a.out - Opens GDB with file a.out, but does not run the program.
r - Runs the program until a breakpoint or error
q - Quit GDB
l - List 10 lines of source code for current line.

Stepping through Code

next - Runs the program until next line, then pauses. If the current line is a function, it executes the entire function , the pauses. next is good for walking through your code quickly.
step - Runs the next instruction, not line.If the current instruction is setting a variable, it is the same as next. If it’s a function, it will jump into the function, execute the first statement, then pause. step is good for diving into the detail of your code.
finish - Finishes executing the current function, then pause (also called step out).

Breakpoints or Watchpoints

print x - Prints current value of variable x.
display x - Constantly displays the value of variable x, which is shown after every step of pause.

Getting Source Code for PAs

Git usage

Use the branch feature of git to manage the process of development. A branch is an ordered list of commits , where a commit refers to some modifications in the project.

git branch - List all branches.
** git checkout -b newbranch** - Create a new branch and check out to it.
git status - See those files modified from the last commit.
git diff - List modifications from the last commit.
git add . - Add the changes to commit.
git commit - Finish a commit.
git checkout branch - Check out to branch.
git log - The changes is traced.

Compiling and Running NEMU

NEMU (NJU Emulator) is a simple but complete full-system emulator designed for teaching purposes. Currently, it supports x86, mips32, riscv32, and riscv64. To build programs run above NEMU, refer to the AM project.

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