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