给系统绘制字体
1.简介
如果我们把字体的大小限定在一个8*16的长方形区域，那么我们在这个区域内，将特定位置的像素点设置成黑色，其他点设置成白色，那么我们就可以得到一个白底黑色的字体： 
2.单字体实现
如果我们把8*16区域当做一个二维数组，白色的像素我们用0表示，黑色像素我们用1表示，
那么上图的字符A, 最顶层的一行全是白色，所以用8个0表示，
第二行，8个像素中，中间两个像素设置成黑色，
于是对应的二进制数就是000 11 000, 对应的16进制数就是0x18,
依次类推，这样对整个字符A, 我们就有可以设置一个对应的数组：
static char fontA[16] = { 0x00, 0x18, 0x18, 0x18,0x18,0x24,0x24,0x24, 0x24, 0x7e, 0x42, 0x42,0x42, 0xe7, 0x00, 0x00 };
拿到字体数组后，绘制时，把数组中的每一个数值取出来，看该数值的二进制形式中，哪一位设置成1，那么就给对应的像素赋予黑色，如果设置成0，就给对应的像素设置成白色，代码如下：
void showFont8(char *vram, int xsize, int x, int y, char c, char* font) { int i; char d; for (i = 0; i < 16; i++) { d = font[i]; if ((d & 0x80) != 0) {vram[(y+i)*xsize + x + 0] = c;} if ((d & 0x40) != 0) {vram[(y+i)*xsize + x + 1] = c;} if ((d & 0x20) != 0) {vram[(y+i)*xsize + x + 2] = c;} if ((d & 0x10) != 0) {vram[(y+i)*xsize + x + 3] = c;} if ((d & 0x08) != 0) {vram[(y+i)*xsize + x + 4] = c;} if ((d & 0x04) != 0) {vram[(y+i)*xsize + x + 5] = c;} if ((d & 0x02) != 0) {vram[(y+i)*xsize + x + 6] = c;} if ((d & 0x01) != 0) {vram[(y+i)*xsize + x + 7] = c;} }
接着 在write_vga_desktop.c的主函数中，在for(;;)死循环前加入一句：
showFont8(vram, xsize, 20, 20, COL8_FFFFFF, fontA);
对其进行一些列的编译、反汇编、汇编、java操作后
编译C文件
i386-elf-gcc -m32 -fno-asynchronous-unwind-tables -s -c -o write_vga_desktop_single_char.o write_vga_desktop_single_char.c
反汇编o文件
./objconv -fnasm write_vga_desktop_single_char.o write_vga_desktop_single_char.asm
删除无用部分
修改kernel
%include "write_vga_desktop_single_char.asm"
修改boot(直接放大一点)直接读了20个扇区 肯定够用了
mov AL, 20 ; AL 表示要练习读取几个扇区
编译boot
nasm -o boot.bat boot.asm
编译kernel
nasm -o kernel.bat kernel.asm
运行java
Load file boot.bat to floppy with cylinder: 0 and sector:1 Load file kernel.bat to floppy with cylinder: 1 and sector:2 Load file kernel.bat to floppy with cylinder: 1 and sector:3 Load file kernel.bat to floppy with cylinder: 1 and sector:4 Load file kernel.bat to floppy with cylinder: 1 and sector:5 Load file kernel.bat to floppy with cylinder: 1 and sector:6 Load file kernel.bat to floppy with cylinder: 1 and sector:7 Load file kernel.bat to floppy with cylinder: 1 and sector:8 Load file kernel.bat to floppy with cylinder: 1 and sector:9 Load file kernel.bat to floppy with cylinder: 1 and sector:10 Load file kernel.bat to floppy with cylinder: 1 and sector:11 Load file kernel.bat to floppy with cylinder: 1 and sector:12 Load file kernel.bat to floppy with cylinder: 1 and sector:13 Load file kernel.bat to floppy with cylinder: 1 and sector:14
运行：
