时间：2009-08-12 点击： 次 来源：本站原创 作者：佚名 - 小 + 大
| 系统引导过程分析与编制 读了2004年《程序员》杂志中的《程序员田园》板块中相关的文章之后，对此颇有感触。因此也想对此作一番小小的研究。 其实说穿了，系统引导很简单： 1、 启动电源之后，硬件完成它所应完成的工作； 2、 BIOS按照CMOS中保存的设置，按照启动顺序读取启动盘中的0柱面0磁道1扇区，将512字节装载到内存的0: 3、 这512字节引导记录的任务就是完成必需的初始化工作，然后装载、运行操作系统的内核。 分析完原理之后，让我们来动动手，作一番尝试。 该过程所需要的工具如下： 1、 NASM或MASM汇编语言编译器，我选择NASM；自从接触到这个Open Source的编译器以后，我就把MASM扔在一旁了。其主要原因，是因为它可以编写跨平台的程序。下载连接：http://sourceforge.net/projects/nasm 2、 一个虚拟机软件，VMWare或者VirtualPC均可。用虚拟机做实验，可免去一次又一次重启的麻烦，也可以保护自己的爱机； 3、 Ultra Edit + ASM语法高亮显示包。用汇编语言写程序，没有什么好的IDE，这个我认为是最完美的组合了。也有人推荐用Edit Plus的，但由于个人喜好的原因，我一直使用这个。http://www.ultraedit.com，词库文件可以免费下载。 先来做一个最简单的测试，它完成的任务仅仅是引导系统后在屏幕上显示红色的’A’： 打开Ultra Edit，输入如下代码：
ORG MOV AH,0 ；调用BIOS中断，设置显示模式 MOV INT 10H
MOV AH,09H ；调用BIOS中断，显示字符 MOV MOV BH,0 MOV BL,4 MOV CX,1 INT 10H
JMP $ ；停止程序运行 DW 0AA55H ；扇区结束标志 然后编译这段程序，nasm boot.asm –f bin –o boot.bin。 -f开关符是指定输出方式为flat，-o是指定输出文件。Flat方式不会为源程序添加任何其它信息，只是照翻源程序，将其翻译成机器码。不过NASM默认的输出方式就是flat，所以这个开关符也可以省略。 因为其不足512字节，所以我用UltraEdit的Hex编辑模式将其打开，插入489个字节，将其凑满512字节。 接下来的任务就是准备一张软盘，用绝对扇区读写工具将其写入磁盘；幸好，手头的MINIX源码光盘中有这么一个工具，可以将其编译一下拿来用，源代码如下： /* fdvol.c - A stripped down version of the MINIX vol program. It takes * a file and breaks into into floppy-sized chunks, writing each one raw * to a floppy. * * Usage: fdvol file drive KB [slow] * * Examples: fdvol * fdvol 1200 b: foo.taz # 1.2 MB floppy b: * fdvol slow * fdvol * * The optional 'slow' parameter forces the program to write in units of 3 * sectors. Folk tradition has it that this works around some buggy BIOSes. * * This code borrows heavily from Mark Becker's RaWrite program. */ #include <sys/types.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <ctype.h> #define TEST 0 #if !TEST #include <alloc.h> #include <bios.h> #include <dir.h> #include <dos.h> #include <io.h> #endif #define FALSE 0 #define TRUE (!FALSE) #define SECTORSIZE 512 #define RESET 0 #define LAST 1 #define READ 2 #define WRITE 3 #define VERIFY 4 #define FORMAT 5 int done; char buffer[18*SECTORSIZE]; /* do I/O in units of up to 18 sectors */ char testbuf[SECTORSIZE]; /* to do a test read of the first sector */ int handler(void) { /* Catch CTRL-C and CTRL-Break. */ done = 1; return(0); } void msg(char (*s)) { /* Print an error message and quit. */ fprintf(stderr, "%s\n", s); _exit(1); } void Error(int status, int cyl, int head, int sector) { /* Identify the error code with a real error message. */ fprintf(stderr, "\nError occured while writing cyl %d, head=%d, sector=%d\n", cyl,head,sector+1); switch (status) { case 0x00: msg("Operation Successful"); break; case 0x01: msg("Bad command"); break; case 0x02: msg("Address mark not found"); break; case 0x03: msg("Attempt to write on write-protected disk"); break; case 0x04: msg("Sector not found"); break; case 0x05: msg("Reset failed (hard disk)"); break; case 0x06: msg("Disk changed since last operation"); break; case 0x07: msg("Drive parameter activity failed"); break; case 0x08: msg("DMA overrun"); break; case 0x09: msg("Attempt to DMA across 64K boundary"); break; case 0x case 0x0B: msg("Bad track detected"); break; case 0x case 0x10: msg("Bad CRC/ECC on disk read"); break; case 0x11: msg("CRC/ECC corrected data error"); break; case 0x20: msg("Controller has failed"); break; case 0x40: msg("Seek operation failed"); break; case 0x80: msg("Attachment failed to respond"); break; case 0xAA: msg("Drive not ready (hard disk only"); break; case 0xBB: msg("Undefined error occurred (hard disk only)"); break; case 0xCC: msg("Write fault occurred"); break; case 0xE0: msg("Status error"); break; case 0xFF: msg("Sense operation failed"); break; } exit(1); } void main(int argc, char *argv[]) { int disknr = 1, fdin, count, drive, head, cyl, status, sector; int max_cyl, chunk, nsectors, ct; long offset, drive_size, r, cyl_size; char *p, c; int slow; int kbsize; char **files; int nfiles, i; #if !TEST /* Catch breaks. */ ctrlbrk(handler); #endif #if 0 /* Do we have to clear the screen? */ fprintf(stderr, "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n"); #endif if (argc > 1 && strcmp(argv[1], "slow") == 0) { /* Lousy BIOS? */ slow = 1; argc--; argv++; } else { slow = 0; } /* Check the arguments for validity. */ if (argc < 4) msg("Usage: fdvol [slow] #kilobytes drive-letter file1 [file2 ...]"); kbsize = atoi(argv[1]); p = argv[2]; c = *p; if (c == 'a' || c == 'A') drive = 0; else if (c == 'b' || c == 'B') drive = 1; else msg("fdvol: Second parameter must be drive, either a: or b:"); files = argv + 3; nfiles = argc - 3; switch(kbsize) { case 360: cyl_size = 9*2*SECTORSIZE; /* bytes/cylinder */ max_cyl = 39; /* zero-based counting */ drive_size = cyl_size * (max_cyl+1); chunk = (!slow ? 9 * SECTORSIZE : 3 * SECTORSIZE); nsectors = chunk/SECTORSIZE; break; case 720: cyl_size = 9*2*SECTORSIZE; /* bytes/cylinder */ max_cyl = 79; /* zero-based counting */ drive_size = cyl_size * (max_cyl+1); chunk = (!slow ? 9 * SECTORSIZE : 3 * SECTORSIZE); nsectors = chunk/SECTORSIZE; break; case 1200: cyl_size = 15*2*SECTORSIZE; /* bytes/cylinder */ max_cyl = 79; /* zero-based counting */ drive_size = cyl_size * (max_cyl+1); chunk = (!slow ? 15 * SECTORSIZE : 3 * SECTORSIZE); nsectors = chunk/SECTORSIZE; break; case 1440: cyl_size = 18*2*SECTORSIZE; /* bytes/cylinder */ max_cyl = 79; /* zero-based counting */ drive_size = cyl_size * (max_cyl+1); chunk = (!slow ? 18 * SECTORSIZE : 3 * SECTORSIZE); nsectors = chunk/SECTORSIZE; break; default: msg("fdvol: First parameter must be one of: 360, 720, 1200, or 1440"); } #if !TEST biosdisk(RESET, drive, 0, 0, 0, 0, testbuf); #endif /* * Start writing data to diskette until there is no more data to write. * Optionally read and write in units of 3 sectors. Folk tradition says * that this makes fewer buggy BIOSes unhappy than doing a whole track at a * time. */ offset = 0; i = 0; fdin = -1; while(1) { if (done > 0) { if (done == 1) msg("User abort"); #if !TEST biosdisk(READ, drive, 0, 0, 1, 1, testbuf); /* Retract head */ #endif fprintf(stderr, "Done. \n"); exit(done == 1 ? 1 : 0); } /* Until a chunk is read. */ count = 0; while (count < chunk) { if (fdin == -1) { /* open next file */ #if !TEST _fmode = O_BINARY; #endif fdin = open(files[i], O_RDONLY); if (fdin < 0) { perror(files[i]); exit(1); } } /* read from file */ ct = read(fdin, buffer + count, chunk - count); if (ct < 0) { perror(files[i]); exit(1); } if (ct == 0) { /* end of file */ close(fdin); fdin = -1; /* choose next file */ if (++i >= nfiles) break; /* no more files */ } count += ct; } if (count == 0) { /* absolute EOF */ done = 2; continue; } if (count < chunk) { /* pad last track */ /* Pad out buffer with zeroes. */ p = &buffer[count]; while (p < &buffer[chunk]) *p++ = 0; done = 2; } r = offset % drive_size; if (r == 0) { /* An integral number of diskettes have been filled. Prompt. */ fprintf(stderr, "Please insert formatted diskette #%d in drive %c, then hit Enter%c\n", disknr, c, 7); disknr++; #if !TEST while(bioskey(1) == 0) ; /* wait for input */ if ((bioskey(0) & 0x biosdisk(READ, drive, 0, 0, 1, 1, testbuf); /* get it going */ #endif } /* Compute cyl, head, sector. */ cyl = r/cyl_size; r -= cyl * cyl_size; head = (r < cyl_size/2 ? 0 : 1); r -= head * cyl_size/2; sector = r/SECTORSIZE;
fprintf(stderr, "Track: %2d Head: %d Sector: %2d File offset: %ld\r", cyl, head, sector+1,offset); #if !TEST status = biosdisk(WRITE, drive, head, cyl, sector+1, nsectors, buffer); if (status != 0) Error(status, cyl, head, sector); #else write(1, buffer, chunk); #endif offset += chunk; } } 然后，用这个工具将刚才编译成功的bin文件写入软盘就成功了。J 如果要对其进行更深入的研究，可以参阅相关书籍。 在http://sourceforge.net/上面搜索NASM的时候，我偶然看到了一个FDOS的项目，可以免费获得。它是“在一张磁盘上的DOS系统”，支持常用DOS命令、FAT12、TSR等。它的文档中有很详细的引导记录说明，我将在下一篇中对其源代码作一些剖析。 另外，网上也能找到MSDOS系统的汇编源代码，有兴趣的同志也可以拿来研究一下。当然，拿Linux的引导代码来研究也未尝不可，但刚开始的时候，不应好高骛远，打击自己的信心，还是从简单的开始吧。 |
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