深入理解 GNU GRUB - 03 diskboot.S 3.3 diskboot.S详细注释

转载注明出处(cppgp: http://blog.csdn.net/cppgp)

 

diskboot.S位于grub-1.98/boot/i386/pc/目录，采用AT&T汇编语法编写。详细注释如下：

 

/* * GRUB -- GRand Unified Bootloader * Copyright (C) 1999,2000,2001,2002,2006,2007,2009,2010 Free Software Foundation, Inc. * * GRUB is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * GRUB is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with GRUB. If not, see <http://www.gnu.org/licenses/>. *//* * cppgp 注释 * * 转载请注明原作者 * * 日期: 2011-04-22 * * Email * [e-mail], * [e-mail] * * GRUB version: * gnu grub-1.98 */ /* * diskboot.S * * diskboot.S生成diskboot.img, 共512字节 * * 安装程序根据实际情况, 改写blocklist数据集. 最多支持 * 15个数据集, 每个数据集占用12字节, 0~7字节表示起始 * 扇区， 8~9字节表示该数据集扇区数, 10~11表示目的 * 缓冲区起始段地址. 必须有一个数据集的扇区数字段 * 为0, 程序根据此判断读取结束. * * * 安装在硬盘上时,DPT部分(0x1BE~0x1FD)保留不变, * * 安装在软盘上时, DPT部分(0x1BE~0x1FD)是软盘复位和 * 扇区探测代码. 扇区末尾两字节 * * 扇区最后两字节 写入0xAA55 (小端表示, 0x1FE位置为0x55, * 0x1FF位置为0xAA). * * diskboot.img开机时加载到0x8000~0x81FF, 并以CS:IP=0x0000:0x8000 * 跳转执行, 它将加载kernel.img到内存0x8200开始位置,并以 * CS:IP=0x0000:0x8200跳转执行. kernel.img扇区数可能有多个, * 在生成影像(grub-mkinage)时确定. * * diskboot.img可以使用boot.img设置的堆栈和寄存器值,但是在 * 跳转到0x8200之前,要确保没有更改这些设置, 因为kernel.img * 还会用到这些寄存器. * * diskboot.img当前只用到一个数据集. 在我的机器上, 反解 * 安装以后的diskboot.img, 得到的值如下: * * 起始扇区: 0x0000 0000 0000 0002 * 扇区数 : 0x002e * 段地址 : 0x0820 */#include <grub/symbol.h>#include <grub/machine/boot.h>/* * defines for the code go here */#define MSG(x)movw $x, %si; call LOCAL(message).file"diskboot.S".text/* Tell GAS to generate 16-bit instructions so that this code works in real mode. *//* * 告诉汇编器产生16位代码 */.code16/* * 如同在boot.S所指明的, * 此处的代码被加载在内存0x8000处 * 并且以CS:IP=0x0000:0x8000跳转执行 */.globlstart, _startstart:_start:/* * _start is loaded at 0x2000 and is jumped to with * CS:IP 0:0x2000 in kernel. *//* * we continue to use the stack for boot.img and assume that * some registers are set to correct values. See boot.S * for more information. *//* * boot.img中已设置堆栈(SS:SP=0x0000:0x2000) * 并且假定一些寄存器被设置为正确值 * 其中包括: * DL: 引导驱动器 * SI: DAP地址(BPB数据块, * -1(%si)可获取读取模式LBA/CHS) * (%si)LBA读的DAP参数起始地址 * 或CHS参数存放地址 * DS=SS=0: 数据段/堆栈段地址 *//* save drive reference first thing! *//* 驱动器压栈 */pushw%dx/* print a notification message on the screen *//* * 向终端输出提示信息"loading" * notification_string="loading" */pushw%siMSG(notification_string)popw%si/* this sets up for the first run through "bootloop" *//* * GRUB_BOOT_MACHINE_LIST_SIZE=12 * (fitstlist-12) ~ firstlist共12字节空间, 指定GRUB内核起始扇区,扇区数,拷贝时的代码段 * 起始扇区8字节, 扇区数2字节,代码段2字节 * 代码中的标签分别是:blocklist_default_start, blocklist_default_len, blocklist_default_seg * blocklist_default_start默认值2，在安装时候指定(grub-install), 决定GRUB内核起始扇区的LBA地址 * blocklist_default_len默认值0, 在制作引导镜像时候生成 (grub-mkimage), 设定为正确的GRUB内核长度 * blocklist_default_seg跳转到GRUB内核时的默认代码段值, 默认值CS=0x820, * 它仅仅在LOCAL(copy_buffer)中用到, 最终跳转时, 使用CS:IP=0x0000:0x8200 (blocklist_default_seg<<4) */movw$(firstlist - GRUB_BOOT_MACHINE_LIST_SIZE), %di/* save the sector number of the second sector in %ebp *//* * 8字节起始扇区的低地址(%si)保存到ESP寄存器 * LBA寻址用到高4字节. * CHS寻址,如果高4字节不为0生成错误,因为CHS寻址不可达(CHS寻址范围7.88GiB, 参考2.1节) */movl(%di), %ebp /* this is the loop for reading the rest of the kernel in *//* 加载所有数据集, 当前只使用一个数据集*/LOCAL(bootloop):/* check the number of sectors to read *//* * 8(%di)确定GRUB内核扇区数 * grub-mkimage生成镜像时改写为正确的内核长度 * 每次循环加载减去已读取的扇区 * 为0时表示已经加载完毕, 跳转到LOCAL(bootit) */cmpw$0, 8(%di)/* if zero, go to the start function *//* 所有数据集加载完毕跳转到LOCAL(bootit) */jeLOCAL(bootit)/* LOCAL(setup_sectors)只加载当前数据集 */LOCAL(setup_sectors):/* check if we use LBA or CHS *//* * boot.img中已经设置读取模式: 1表示LBA, 0表示CHS */cmpb$0, -1(%si)/* use CHS if zero, LBA otherwise *//* CHS模式读 */jeLOCAL(chs_mode)/* load logical sector start */movl(%di), %ebxmovl4(%di), %ecx/* the maximum is limited to 0x7f because of Phoenix EDD *//* Phoenix EDD限制单次读不能超过0x7F扇区 */xorl%eax, %eaxmovb$0x7f, %al/* how many do we really want to read? */cmpw%ax, 8(%di)/* compare against total number of sectors *//* which is greater? */jg1f/* if less than, set to total *//* 如果到达这里,表示剩余扇区不足0x7F扇区,这次就可以加载完毕了 */movw8(%di), %ax1:/* subtract from total *//* 减去这次要读取的扇区数, 代码用8(%di) 是否为0来判断有否加载完毕 */subw%ax, 8(%di)/* add into logical sector start *//* * 起始扇区要加上这次将要读取的扇区数 * 注意是8字节的起始扇区 * 高4字节的加要注意进位 */addl%eax, (%di)adcl$0, 4(%di)/* set up disk address packet *//* * 设置LBA读参数并调用BIOS中断 * 参考boot.S LBA读注释 *//* the size and the reserved byte */movw$0x0010, (%si)/* the number of sectors */movw%ax, 2(%si)/* the absolute address */movl%ebx, 8(%si)movl%ecx, 12(%si)/* the segment of buffer address */movw$GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)/* save %ax from destruction! *//* 保存这次要读取的扇区数 */pushw%ax/* the offset of buffer address */movw$0, 4(%si)/* * BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory *Call with%ah = 0x42 *%dl = drive number *%ds:%si = segment:offset of disk address packet *Return: *%al = 0x0 on success; err code on failure */movb$0x42, %ahint$0x13jcLOCAL(read_error)movw$GRUB_BOOT_MACHINE_BUFFER_SEG, %bxjmpLOCAL(copy_buffer)LOCAL(chs_mode):/* * CHS读 * 判断C/H/S参数是否越界 * 调用CHS读的BIOS中断 * 参考boot.S CHS读注释 *//* load logical sector start (top half) */movl4(%di), %eaxorl%eax, %eaxjnzLOCAL(geometry_error)/* load logical sector start (bottom half) */movl(%di), %eax/* zero %edx */xorl%edx, %edx/* divide by number of sectors */divl(%si)/* save sector start */movb%dl, 10(%si)xorl%edx, %edx/* zero %edx */divl4(%si)/* divide by number of heads *//* save head start */movb%dl, 11(%si)/* save cylinder start */movw%ax, 12(%si)/* do we need too many cylinders? */cmpw8(%si), %axjgeLOCAL(geometry_error)/* determine the maximum sector length of this read */movw(%si), %ax/* get number of sectors per track/head *//* subtract sector start */subb10(%si), %al/* how many do we really want to read? */cmpw%ax, 8(%di)/* compare against total number of sectors *//* which is greater? */jg2f/* if less than, set to total */movw8(%di), %ax2:/* subtract from total */subw%ax, 8(%di)/* add into logical sector start */addl%eax, (%di)adcl$0, 4(%di)/* * This is the loop for taking care of BIOS geometry translation (ugh!) *//* get high bits of cylinder */movb13(%si), %dlshlb$6, %dl/* shift left by 6 bits */movb10(%si), %cl/* get sector */incb%cl/* normalize sector (sectors gofrom 1-N, not 0-(N-1) ) */orb%dl, %cl/* composite together */movb12(%si), %ch/* sector+hcyl in cl, cylinder in ch *//* restore %dx */popw%dxpushw%dx/* head number */movb11(%si), %dhpushw%ax/* save %ax from destruction! *//* * BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory *Call with%ah = 0x2 *%al = number of sectors *%ch = cylinder *%cl = sector (bits 6-7 are high bits of "cylinder") *%dh = head *%dl = drive (0x80 for hard disk, 0x0 for floppy disk) *%es:%bx = segment:offset of buffer *Return: *%al = 0x0 on success; err code on failure */movw$GRUB_BOOT_MACHINE_BUFFER_SEG, %bxmovw%bx, %es/* load %es segment with disk buffer */xorw%bx, %bx/* %bx = 0, put it at 0 in the segment */movb$0x2, %ah/* function 2 */int$0x13jcLOCAL(read_error)/* save source segment */movw%es, %bxLOCAL(copy_buffer):/* * 无论CHS/LBA, 缓冲区地址为segment:offset=%bx:0x0=0x7000:0x0处 *//* load addresses for copy from disk buffer to destination *//* 设置目的段 */movw10(%di), %es/* load destination segment *//* restore %ax *//* * 调用BIOS中断前已经把这次要读取的扇区数压栈 * 每个扇区512字节即2^9 * 然后%ax左移5位并加到段地址上, * 段地址要左移4位放到地址线上, * 因此,等价于%ax左移9位放到地址线上 * 这正好是一个扇区数据占用的内存空间 */popw%ax/* determine the next possible destination address (presuming512 byte sectors!) *//* * 下一次可能的目的段地址(强制要求512字节扇区) * 本次使用的目的段在此之前已经保存到%es了 */shlw$5, %ax/* shift %ax five bits to the left */addw%ax, 10(%di)/* add the corrected value to the destination address for next time *//* save addressing regs */pushapushw%ds/* get the copy length *//* * %ax左移3位, 共左移8位 * 因此%ax中是这次读取的字数(word, 1-word=2-byte) * 设置%cx为需要拷贝的字数,rep使用%cx作为循环计数器 */shlw$3, %axmovw%ax, %cx/* * 以扇区为单位,源/目的串偏移量都是0 */xorw%di, %di/* zero offset of destination addresses */xorw%si, %si/* zero offset of source addresses *//* 进入LOCAL(copy_buffer)之前, %bx持有缓冲区段地址 */movw%bx, %ds/* restore the source segment *//* 字符串操作指针移动方向为前向 */cld/* sets the copy direction to forward *//* perform copy *//* 完成拷贝 */rep/* sets a repeat */movsw/* this runs the actual copy *//* restore addressing regs and print a dot with correct DS (MSG modifies SI, which is saved, and unused AX and BX) *//* MSG不用到DS,/* * 设置正确的数据段地址%ds * 调用MSG显示"." * 对于慢速存储设备, * 提供用户反馈, 以免被当做死机 */popw%dsMSG(notification_step)popa/* check if finished with this dataset *//* 检测是否读取完毕, 8(%di)持有需要读取的剩余扇区数 */cmpw$0, 8(%di)jneLOCAL(setup_sectors)/* update position to load from *//* * 更新为上一个数据集 * 意指(firstlist-(x+1)*12)~(firstlist-x*12)位置, x从0开始递增 * 可以有多个数据集 * 但是不能和LOCAL(message)重叠 * 多个数据集数据最终是否连续, 依赖于该数据集合上一个数据集 * 缓冲区段地址 * * 否则diskboot会出错 * 反汇编显示LOCAL(message)结束处地址0x8147 * 因此0x148~0x1FF可全部用作数据集 * 每个数据集12字节,最多可有(0x200-0x148)/12=15个数据集 * 最开始处数据集扇区数必须为0 (否则重叠到LOCAL(message)去啦) * ==> 最多可有14个有效数据集 * 反汇编安装后的diskboot.img的结果, 显示只用了一个数据集 * * 推导: * 每个数据集采用2字节描述扇区数,因此不能超过0xFFFF个扇区 * 14个有效数据集位置, 共计扇区数0xFFFF*14 * 共计字节数= 0xFFFF*14*512=469754880bytes=447MB远超实模式下的寻址空间了 */subw$GRUB_BOOT_MACHINE_LIST_SIZE, %di/* jump to bootloop *//* 跳转到bootloop处理下一数据集 */jmpLOCAL(bootloop)/* END OF MAIN LOOP *//* * 所有数据读取完毕 * 单个数据集内的数据总是连续放置在内存中 * 当前只使用一个数据集,放置在0x8200起始的内存中 * LOCAL(bootit)打印提示信息并跳转执行 * GRUB_BOOT_MACHINE_KERNEL_ADDR=0x8000 * CS:IP=0x0000:0x8200 * startup.S生成的指令开始执行 */LOCAL(bootit):/* print a newline */MSG(notification_done)popw%dx/* this makes sure %dl is our "boot" drive */ljmp$0, $(GRUB_BOOT_MACHINE_KERNEL_ADDR + 0x200)/* * 错误提示字符串和输出函数 * 参考boot.S注释 *//* * BIOS Geometry translation error (past the end of the disk geometry!). */LOCAL(geometry_error):MSG(geometry_error_string)jmpLOCAL(general_error)/* * Read error on the disk. */LOCAL(read_error):MSG(read_error_string)LOCAL(general_error):MSG(general_error_string)/* go here when you need to stop the machine hard after an error condition */LOCAL(stop):jmpLOCAL(stop)notification_string:.asciz "loading"notification_step:.asciz "."notification_done:.asciz "/r/n"geometry_error_string:.asciz "Geom"read_error_string:.asciz "Read"general_error_string:.asciz " Error"/* * message: write the string pointed to by %si * * WARNING: trashes %si, %ax, and %bx *//* * Use BIOS "int 10H Function 0Eh" to write character in teletype mode *%ah = 0xe%al = character *%bh = page%bl = foreground color (graphics modes) */1:movw$0x0001, %bxmovb$0xe, %ahint$0x10/* display a byte */incw%siLOCAL(message):movb(%si), %alcmpb$0, %aljne1b/* if not end of string, jmp to display */ret/* * 反汇编结果显示, 此处地址是0x8148 * 可在此处添加. = _start + 0x148 测试 * * 0x148~0x1FF共计184字节,可存放15个12字节的数据集 * 按照从下到上的顺序存放数据集 * 最后一个有效数据集的扇区数(数据集内偏移量8)必须为0 */ /* * This area is an empty space between the main body of code below which * grows up (fixed after compilation, but between releases it may change * in size easily), and the lists of sectors to read, which grows down * from a fixed top location. */.word 0.word 0. = _start + 0x200 - GRUB_BOOT_MACHINE_LIST_SIZE /* fill the first data listing with the default */blocklist_default_start:/* this is the sector start parameter, in logical sectors from the start of the disk, sector 0 */.long 2, 0blocklist_default_len:/* this is the number of sectors to read. grub-mkimage will fill this up */.word 0blocklist_default_seg:/* this is the segment of the starting address to load the data into */.word (GRUB_BOOT_MACHINE_KERNEL_SEG + 0x20)firstlist:/* this label has to be after the list data!!! */