让系统内核突破512字节的限制

之前做的系统内核，放在虚拟软盘的第一扇区，由于一个扇区只有512字节，因此系统内核的大小无法超过512字节。但是，一个拥有完整功能的内核不可能只有512字节，因此要想越过512字节的限制，具体做法就是做一个内核加载器（大小小于512字节），放入第一扇区，加载器加载入内存后，再将内核从软盘加载到系统内存，最后跳转到内核的加载地址

假设把编译好的内核代码写入软盘的第一柱面，第2扇区，那么加载器的代码如下（boot.asm）

org  0x7c00;

LOAD_ADDR  EQU  0X8000

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

readFloppy:
    mov  CH, 1  ;CH 用来存储柱面号
    mov  DH, 0  ;DH 用来存储磁头号
    mov  CL, 2  ;CL 用来存储扇区号

    mov  BX, LOAD_ADDR  ;ES:BX 数据存储缓冲区

    mov  AH, 0x02  ;AH = 02 表示要做的是读盘操作
    mov  AL, 1  ;AL表示要练习读取几个扇区
    mov  DL, 0  ;驱动器编号，一般我们只有一个软盘驱动器，所以写死为0
    INT  0x13 ;调用BIOS中断实现磁盘读取功能

    JC  fin

    jmp  LOAD_ADDR

fin:
    HLT
    jmp  fin

readFloppy 这段代码的作用是从软盘的1柱面2扇区，将内核读取到系统内存的0x8000处，读取成功后，通过一个 jmp 跳转到内核的加载地址，将机器的控制权转交给内核。内核代码如下（kernel.asm）

org 0x8000

entry:
    mov ax,0
    mov ss,ax
    mov ds,ax
    mov es,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 "This is Hello World from kernel"

下载 nasm软件 对汇编语言进行编译。使用命令分别编译加载器和内核：

nasm boot.asm -o boot.bat 
nasm kernel.asm -o kernel.bat

编译好以后，通过 java 代码将编译好的内核写入虚拟软盘

Floppy.java

import java.io.DataOutputStream;
import java.io.FileOutputStream;
import java.util.ArrayList;
import java.util.HashMap;

public class Floppy {
    enum MAGNETIC_HEAD {
        MAGNETIC_HEAD_0, MAGETIC_HEAD_1
    };

    public int SECTOR_SIZE = 512;
    private int CYLINDER_COUNT = 80; // 80个柱面
    private int SECTORS_COUNT = 18;
    private MAGNETIC_HEAD magneticHead = MAGNETIC_HEAD.MAGNETIC_HEAD_0;
    private int current_cylinder = 0;
    private int current_sector = 0;

    private HashMap<Integer, ArrayList<ArrayList<byte[]>>> floppy = new HashMap<Integer, ArrayList<ArrayList<byte[]>>>(); // 一个磁盘两个面

    public Floppy() {
        initFloppy();
    }

    private void initFloppy() {
        // 一个磁盘有两个盘面
        floppy.put(MAGNETIC_HEAD.MAGNETIC_HEAD_0.ordinal(), initFloppyDisk());
        floppy.put(MAGNETIC_HEAD.MAGETIC_HEAD_1.ordinal(), initFloppyDisk());
    }

    private ArrayList<ArrayList<byte[]>> initFloppyDisk() {
        ArrayList<ArrayList<byte[]>> floppyDisk = new ArrayList<ArrayList<byte[]>>(); // 磁盘的一个面
        // 一个磁盘面有80个柱面
        for (int i = 0; i < CYLINDER_COUNT; i++)
            floppyDisk.add(initCylinder());
        return floppyDisk;
    }

    private ArrayList<byte[]> initCylinder() {
        // 构造一个柱面，一个柱面有18个扇区
        ArrayList<byte[]> cylinder = new ArrayList<byte[]>();
        for (int i = 0; i < SECTORS_COUNT; i++) {
            byte[] sector = new byte[SECTOR_SIZE];
            cylinder.add(sector);
        }
        return cylinder;
    }

    public void setMagneticHead(MAGNETIC_HEAD head) {
        magneticHead = head;
    }

    public void setCylinder(int cylinder) {
        if (cylinder < 0)
            this.current_cylinder = 0;
        else if (cylinder >= 80)
            this.current_cylinder = 79;
        else
            this.current_cylinder = cylinder;
    }

    public void setSector(int sector) {
        // sector 编号从1到18
        if (sector < 0)
            this.current_sector = 0;
        else if (sector > 18)
            this.current_sector = 18 - 1;
        else
            this.current_sector = sector - 1;
    }

    public byte[] readFloppy(MAGNETIC_HEAD head, int cylinder_num, int sector_num) {
        setMagneticHead(head);
        setCylinder(cylinder_num);
        setSector(sector_num);

        ArrayList<ArrayList<byte[]>> disk = floppy.get(this.magneticHead.ordinal());
        ArrayList<byte[]> cylinder = disk.get(this.current_cylinder);

        byte[] sector = cylinder.get(this.current_sector);

        return sector;
    }

    public void writeFloppy(MAGNETIC_HEAD head, int cylinder_num, int sector_num, byte[] buf) {
        setMagneticHead(head);
        setCylinder(cylinder_num);
        setSector(sector_num);

        ArrayList<ArrayList<byte[]>> disk = floppy.get(this.magneticHead.ordinal());
        ArrayList<byte[]> cylinder = disk.get(this.current_cylinder);

        byte[] buffer = cylinder.get(this.current_sector);
        System.arraycopy(buf, 0, buffer, 0, buf.length);
    }

    public void makeFloppy(String fileName) {
        try {
            DataOutputStream out = new DataOutputStream(new FileOutputStream(fileName));
            for (int cylinder = 0; cylinder < CYLINDER_COUNT; cylinder++)
                for (int head = 0; head <= MAGNETIC_HEAD.MAGETIC_HEAD_1.ordinal(); head++)
                    for (int sector = 1; sector <= SECTORS_COUNT; sector++) {
                        byte[] buf = readFloppy(MAGNETIC_HEAD.values()[head], cylinder, sector);
                        out.write(buf);
                    }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

OPeratingSystem.java

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

public class OperatingSystem {

    private Floppy floppyDisk = new Floppy();
    private int MAX_SECTOR_NUM = 18;

    private void writeFileToFloppy(String fileName, boolean bootable, int cylinder, int beginSec) {
        File file = new File(fileName);
        InputStream in = null;

        try {
            in = new FileInputStream(file);
            byte[] buf = new byte[512];
            if (bootable) {
                buf[510] = 0x55;
                buf[511] = (byte) 0xaa;
            }

            while (in.read(buf) > 0) {
                // 将内核读入到磁盘第0面，第0柱面，第1个扇区
                floppyDisk.writeFloppy(Floppy.MAGNETIC_HEAD.MAGNETIC_HEAD_0, cylinder, beginSec, buf);
                beginSec++;

                if (beginSec > MAX_SECTOR_NUM) {
                    beginSec = 1;
                    cylinder++;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
            return;
        }
    }

    public OperatingSystem(String s) {
        writeFileToFloppy(s, true, 0, 1);
    }

    public void makeFllopy() {
        writeFileToFloppy("kernel.bat", false, 1, 2);
        floppyDisk.makeFloppy("system.img");
    }

    public static void main(String[] args) {
        OperatingSystem op = new OperatingSystem("boot.bat");
        op.makeFllopy();
    }
}

运行的是 OperatingSystem.java 代码。将生成的虚拟软盘文件system.img加载到虚拟机中启动，就可以看到如下效果