MacOS环境-手写操作系统-01-最小操作系统原创

1.简介

文章写于两年前的 MacBookAir(2015)
目前笔者为 MacBookPro M1 (抽查了部分都运行正常)
Github项目地址: https://github.com/wdkang123/MyOperatingSystem
MacOS X86架构（x新版的arm架构的我没有所以大家自行测试)
VirtualBox
C/C++环境（Xcode必装）

由之前的最小内核将其转换为二进制再成为十六进制

private int[] imgContent = new int[]{
     0xeb,0x4e,0x90,0x48,0x45,0x4c,0x4c,0x4f,0x49,0x50,0x4c,0x00,0x02,0x01,0x01,0x00,0x02,0xe0,        0x00,0x40,0x0b,0xf0,0x09,0x00,0x12,0x00,0x02,0x00,0x00,0x00,0x00,0x00,0x40,0x0b,0x00,0x00,0x00,0x00,0x29,
    0xff,0xff,0xff,0xff,0x48,0x45,0x4c,0x4c,0x4f,0x2d,0x4f,0x53,0x20,0x20,0x20,0x46,0x41,0x54,0x31,0x32,        0x20,0x20,0x20,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xb8,0x00,0x00,0x8e,
        0xd0,0xbc,0x00,0x7c,0x8e,0xd8,0x8e,0xc0,0xbe,0x74,0x7c,0x8a,
     0x04,0x83,0xc6,0x01,0x3c,0x00,0x74,0x09,0xb4,0x0e,0xbb,0x0f,0x00,0xcd,0x10,0xeb,0xee,0xf4,0xeb,0xfd
    };

这段十六进制的代码中设置了一些初始化的数据然后调用bios中断将某个缓冲区中的字符打印到屏幕上去

然后进去了一个死循环实际上这段代码是由汇编实现接着将汇编转换为16进制再将其放到imgContent中

最后让java程序生成虚拟软盘

实际上汇编是这么写的：

org  0x7c00;

jmp  entry
db   0x90
DB   "OSKERNEL"
DW   512
DB   1
DW   1
DB   2
DW   224
DW   2880
DB   0xf0
DW   9
DW   18
DW   2
DD   0
DD   2880
DB   0,0,0x29
DD   0xFFFFFFFF
DB   "MYFIRSTOS  "
DB   "FAT12   "
RESB  18

entry:
    mov  ax, 0
    mov  ss, ax
    mov  ds, ax
    mov  es, ax
    mov  si, msg

putloop:
    mov  al, [si]
    add  si, 1
    cmp  al, 0
    je   fin
    mov  ah, 0x0e
    mov  bx, 15
    int  0x10
    jmp  putloop

fin:
    HLT
    jmp  fin

msg:
    DB    0x0a,  0x0a
    db    "hello, world"
    db    0x0a
    db    0

上面的汇编代码实际上就实现了将hello, world 打印到屏幕上然后进入死循环的功能

1.1 org

上面的程序第一句是 org 0x7c00,

org 的意思是origin 中文意思是“起始起源”

org 后面的7c00 是物理内存地址

假设物理内存是一个byte类型的大数组例如byte[] memory, 如果你有2G内容换算成字节就是2097152 也就相当于memory数组有2097152字节

于是当虚拟机上电然后new一块内存 byte[] memory = new byte[2097152]

org 0x7c00 的意思是将本汇编编译后的二进制数据从memory[0x7c00]处写入memory.

1.2 jmp

jmp entry 中的jmp 其实就是c语言中的语句goto, jmp entry 其实是让cpu跳转到entry 处，执行entry下面的代码，如果entry是一个函数名字的话，jmp entry 相当于调用entry函数，类比于java就是函数调用：entry();

1.3 db

从db 0x90 到 RESB 18 这段代码做一些初始化工作

jmp entry 对应的机器代码长度是3字节

那么db 0x90 的意思就是 memory[0x7c00+3] = 0x90 也就是说db 0x90 实际上做的是赋值操作

db 0x90 表示将给定位置处的一个字节赋予数值0x90 赋值的内存位置就在0x7c00+3处

1.4 msg:

msg:
DB 0x0a, 0x0a
db “hello, world”
db 0x0a

相当于 C语言的 char* msg = “\n\nhello,world\n”

1.5 int

调用一个中断中断其实就是一个函数调用

我们在写c语言或java程序时往往需要调用一些系统库函数例如printf, 或java的System.out.print.

中断就是bios提供给汇编语言的库函数，这些库函数都放入到一个数组里，int 0x10 意思是在库函数数组中取出第0x10个库函数，然后执行该库函数的代码

1.6 put loop:

putloop:
mov al, [si]
add si, 1
cmp al, 0
je fin
mov ah, 0x0e
mov bx, 15
int 0x10
jmp putloop

就相当于C语言：
do {
char al = *si;
si++;
if (al == 0) {
goto fin
}

printf(“%c”, al);
} while(true);

2.进行编译

将这段代码进行汇编:

nasm boot.asm -o boot.bat

得到了 boo.bat

b800 008e d08e d88e c0be 217c 8a04 83c6
013c 0074 09b4 0ebb 0f00 cd10 ebee f4eb
fd0a 0a68 656c 6c6f 2c20 776f 726c 640a
00

将这段放入到java工程中的imgContent中去

public class OperatingSystem {
    private int[] imgContent = new int[]{
        0xeb,0x4e,0x90,0x48,0x45,0x4c,0x4c,0x4f,0x49,0x50,0x4c,0x00,0x02,0x01,0x01,0x00,0x02,0xe0,
        0x00,0x40,0x0b,0xf0,0x09,0x00,0x12,0x00,0x02,0x00,0x00,0x00,0x00,0x00,0x40,0x0b,0x00,0x00,0x00,0x00,0x29,
        0xff,0xff,0xff,0xff,0x48,0x45,0x4c,0x4c,0x4f,0x2d,0x4f,0x53,0x20,0x20,0x20,0x46,0x41,0x54,0x31,0x32,
        0x20,0x20,0x20,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xb8,0x00,0x00,0x8e,
        0xd0,0xbc,0x00,0x7c,0x8e,0xd8,0x8e,0xc0,0xbe,0x74,0x7c,0x8a,
        0x04,0x83,0xc6,0x01,0x3c,0x00,0x74,0x09,0xb4,0x0e,0xbb,0x0f,0x00,0xcd,0x10,0xeb,0xee,0xf4,0xeb,0xfd
    };

    private ArrayList<Integer> imgByteToWrite = new ArrayList<Integer>();

    private void readKernelFromFile(String fileName) {
        File file = new File(fileName);
        InputStream in = null;

        try {
            in = new FileInputStream(file);
            int tempbyte;
            while ((tempbyte = in.read()) != -1) {
                imgByteToWrite.add(tempbyte);
            }
        } catch(IOException e) {
            e.printStackTrace();
            return;
        }

        imgByteToWrite.add(0x55);
        imgByteToWrite.add(0xaa);
        imgByteToWrite.add(0xf0);
        imgByteToWrite.add(0xff);
        imgByteToWrite.add(0xff);

    }

    public OperatingSystem(String s) {
    /*  for (int i = 0; i < imgContent.length; i++) {
            imgByteToWrite.add(imgContent[i]);
        }

        imgByteToWrite.add(0x0a);
        imgByteToWrite.add(0x0a);
        for (int j = 0; j < s.length(); j++) {
            imgByteToWrite.add((int)s.charAt(j));
        }
        imgByteToWrite.add(0x0a);

        int len = 0x1fe;
        int curSize = imgByteToWrite.size();
        for (int k = 0; k < len - curSize; k++) {
            imgByteToWrite.add(0);
        }

        //0x1fe-0x1f: 0x55, 0xaa
        //0x200-0x203: f0 ff  ff
        imgByteToWrite.add(0x55);
        imgByteToWrite.add(0xaa);
        imgByteToWrite.add(0xf0);
        imgByteToWrite.add(0xff);
        imgByteToWrite.add(0xff);
      */

        readKernelFromFile("boot.bat");

        int len = 0x168000;
        int curSize = imgByteToWrite.size();
        for (int l = 0; l < len - curSize; l++) {
            imgByteToWrite.add(0);
        }

    }

    public void makeFllopy()   {
        try {
            DataOutputStream out = new DataOutputStream(new FileOutputStream("system.img"));
            for (int i = 0; i < imgByteToWrite.size(); i++) {
                out.writeByte(imgByteToWrite.get(i).byteValue());
            }
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public static void main(String[] args) {
        OperatingSystem op = new OperatingSystem("hello, this is my first line of my operating system code");
        op.makeFllopy();
    }
}