全部博文（290）
分类： LINUX
2010-11-14 22:53:25
/*预处理包含定义
*其中STAGE1_5被自GRUB2后不使用
*用到的share.h被其它一些源文件使用，
*此文件使用到的宏不多，用到时给与说明
******************************/
#define ASM_FILE
#include
#ifndef STAGE1_5
#include
#endif
/*
* defines for the code go here
*/
/* Absolute addresses
This makes the assembler generate the address without support
from the linker. (ELF can't relocate 16-bit addresses!) */
/*重定位，并根据是否定义STAGE1_5选择装入地址*/
#ifdef STAGE1_5
# define ABS(x) (x-_start+0x2000)
#else
# define ABS(x) (x-_start+0x8000)
#endif /* STAGE1_5 */
/* 调用int 10h中断以打印机模式在屏幕上显示一字符串*/
#define MSG(x) movw $ABS(x), %si; call message
/*****************************************
.file op 指定和目标文件关联的源文件名
*****************************************/
.file "start.S"
/*****************************************
.text代码段声明; .code16使用实模式
*****************************************/
.text
/* 告诉GAS产生16位指令，以便此代码能在实模式下运行 */
.code16
.globl start, _start
start:
_start:
/*start由stage1载入到0x8000处，si仍然用于DiskAddressPacket结构
*
/* 首先保存设备类型号，stage1由bios初始化在dl中! */
pushw %dx
/* 在屏幕上打印字符串：loading stage2 or loading stage1_5
*由于在输出字符的时候使用了si寄存器，故此保存原si地址
*字符串输出完毕后恢复原字符串*/
pushw %si
MSG(notification_string)
popw %si
/*** #define BOOTSEC_LISTSIZE 8" firstlist为此段末尾偏移地址*/
movw $ABS(firstlist - BOOTSEC_LISTSIZE), %di
/* LBA以0开始，故此，stage1占1扇区，start占一扇区，故stage2从2开始，并存 倒数第8个字节处。 */
movl (%di), %ebp
/*由于firstlist -08h处保存着需要载入的扇区号和扇区的数目，故此，
* bootloop 子功能，循环将对应的扇区号中的数据载入相应内存中并执行，
*可能有人会问，为什么不将stage1和start融合在一起呢，这是因为一般的
* 硬盘低格后，默认的扇区大小为512个字节，也就是分开也是不得于而为之
*/
bootloop:
/* [edi]+4处为 .word (STAGE2_SIZE + 511) >> 9，这里不考虑stage1_5的情况
* STAGE2_SIZE为grub编译后的大小，>>9相当于除以512，
*4（%di）处存的是stage2占的扇区数目/
cmpw $0, 4(%di)
/* 如果为0说明使用stage1_5直接跳至bootit处执行 */
je bootit
setup_sectors:
/* stage1中，如果检测支持LBA模式，此语句被执行：movb $1, -1(%si)
* 避免了相同功能的重复调用/
cmpb $0, -1(%si)
/* 结果为0，说明不支持LBA模式 */
je chs_mode
lba_mode:
/* 装载逻辑扇区号入ebx */
movl (%di), %ebx
/* the maximum is limited to 0x7f because of Phoenix EDD
看一下int 43的调用的参数说明：
Format of disk address packet:
Offset Size Description
00h BYTE 10h (size of packet)
01h BYTE reserved (0)
02h WORD number of blocks to transfer (max 007Fh for Phoenix EDD)
04h DWORD -> transfer buffer
08h QWORD starting absolute block number
(for non-LBA devices, compute as
(Cylinder*NumHeads + SelectedHead) * SectorPerTrack +
SelectedSector - 1
--------N-134257DX1234----------------------- */
xorl %eax, %eax
movb $0x7f, %al
/* 比较应该读的数据数否超过最大传输参数 */
cmpw %ax, 4(%di) /* compare against total number of sectors */
/* 如果大于，跳至本句后一个标号 1:处，否则将值付给ax寄存器 */
jg 1f
/* if less than, set to total */
movw 4(%di), %ax
1:
/* 用stage2的总大小减去ax中的值，ax的值为上面的判断后的设置值 */
subw %ax, 4(%di)
/* 将ax的值加上起始扇区号，因为下面的代码会将ax中保存的扇区数目循环读出
* 这样，即使要读的数据超出7f也不用在定义其它的数据来保存剩下的扇区数
* /
addl %eax, (%di)
/* 设置磁盘地址数据包结构体 */
/* the size and the reserved byte */
movw $0x0010, (%si)
/* 需要读出的扇区数目，见到ax的用处了吧！ */
movw %ax, 2(%si)
/* 绝对扇区地址，(low 32 bits) */
movl %ebx, 8(%si)
/* 缓冲区段地址设置，读出的数据保存于此处 #define BUFFERSEG RAW_SEG (0x7000)*/
movw $BUFFERSEG, 6(%si)
/* 保存ax的值 */
pushw %ax
/* zero %eax */
xorl %eax, %eax
/* 缓冲区偏移地址设置 == 0 */
movw %ax, 4(%si)
/* 高32位绝对地址 (high 32 bits) */
movl %eax, 12(%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, %ah
int $0x13
jc read_error
movw $BUFFERSEG, %bx /*缓冲区地址保存*/
jmp copy_buffer /*拷贝读出的数据*/
chs_mode:
/* lBA扇区号送eax寄存器 ，需要将LBA转换为CHS地址在读写*/
movl (%di), %eax
/* edx填零*/
xorl %edx, %edx
/* divide by number of sectors */
divl (%si)
/* save sector start */
movb %dl, 10(%si)
xorl %edx, %edx /* zero %edx */
divl 4(%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? */
cmpw 8(%si), %ax
jge geometry_error
/* determine the maximum sector length of this read */
movw (%si), %ax /* get number of sectors per track/head */
/* subtract sector start */
subb 10(%si), %al
/* how many do we really want to read? */
cmpw %ax, 4(%di) /* compare against total number of sectors */
/* which is greater? */
jg 2f
/* if less than, set to total */
movw 4(%di), %ax
2:
/* subtract from total */
subw %ax, 4(%di)
/* add into logical sector start */
addl %eax, (%di)
/*
* This is the loop for taking care of BIOS geometry translation (ugh!)
*/
/* get high bits of cylinder */
movb 13(%si), %dl
shlb $6, %dl /* shift left by 6 bits */
movb 10(%si), %cl /* get sector */
incb %cl /* normalize sector (sectors go
from 1-N, not 0-(N-1) ) */
orb %dl, %cl /* composite together */
movb 12(%si), %ch /* sector+hcyl in cl, cylinder in ch */
/* restore %dx */
popw %dx
pushw %dx
/* head number */
movb 11(%si), %dh
pushw %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 $BUFFERSEG, %bx
movw %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 $0x13
jc read_error
/* save source segment */
movw %es, %bx
copy_buffer:
/* load addresses for copy from disk buffer to destination */
/*附加段数据初始化。。
* blocklist_default_seg:
* #ifdef STAGE1_5
* .word 0x220
* #else
* .word 0x820 段的开始地址，*16后为 8200，其中8000 - 8200被start占用
******************************************************************** */
movw 6(%di), %es /* load destination segment */
/* 读了几个扇区，吐出来。。 */
popw %ax
/* determine the next possible destination address (presuming
512 byte sectors!) */
//逻辑左移，末尾补零，移1位相当于算术*2，这里以16进制算出下一次拷贝的起始地址；
shlw $5, %ax /* shift %ax five bits to the left */
addw %ax, 6(%di) /* add the corrected value to the destination
address for next time */
/* 环境保存，因为MSG调用int10中断后可能会改变寄存器的值 */
pusha
pushw %ds
/* 由于开始用>>9 ，故此，两次shl 移动的位数为9，从而得到读出的字节数 */
shlw $4, %ax
movw %ax, %cx
xorw %di, %di /* zero offset of destination addresses */
xorw %si, %si /* zero offset of source addresses */
movw %bx, %ds /* 读出后保存于此movw $BUFFERSEG, %bx 缓冲区地址保存
* 这里得分开，BUFFERSEG为0x7000 ，保存到16位的bx中取的是0x70，注意AT&T和MASM的字节序/
cld /* 自增方向标志*/
/* 重复拷贝，每次一个字节，拷贝到0x8200处 */
rep /* sets a repeat */
movsb /* 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) */
popw %ds
MSG(notification_step)
popa /*环境恢复*/
/*检查数据是否已经读完，如果没有，继续读取并拷贝 */
cmpw $0, 4(%di)
jne setup_sectors
/* update position to load from */
subw $BOOTSEC_LISTSIZE, %di
/* jump to bootloop */
jmp bootloop
/* END OF MAIN LOOP */
bootit:
/* 打印\r\n后，恢复寄存器dx的值，根据是否定义了STAGE1_5跳到相应的起始地址处执行装入的stage2的内容 */
MSG(notification_done)
popw %dx /* 保证dl寄存器的内容为boot设置的初始值 */
#ifdef STAGE1_5
ljmp $0, $0x2200
#else /* ! STAGE1_5 */
ljmp $0, $0x8200
#endif /* ! STAGE1_5 */
/*
* BIOS Geometry translation error (past the end of the disk geometry!).
*/
/*错误处理代码，出错后执行，stop ： jmp stop 死循环；*/
geometry_error:
MSG(geometry_error_string)
jmp general_error
/*
* Read error on the disk.
*/
read_error:
MSG(read_error_string)
general_error:
MSG(general_error_string)
/* go here when you need to stop the machine hard after an error condition */
stop: jmp stop
#ifdef STAGE1_5
notification_string: .string "Loading stage1.5"
#else
notification_string: .string "Loading stage2"
#endif
notification_step: .string "."
notification_done: .string "\r\n"
geometry_error_string: .string "Geom"
read_error_string: .string "Read"
general_error_string: .string " 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, %bx
movb $0xe, %ah
int $0x10 /* display a byte */
incw %si
message:
movb (%si), %al
cmpb $0, %al
jne 1b /* if not end of string, jmp to display */
ret
lastlist:
/*
* 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 - BOOTSEC_LISTSIZE
/* fill the first data listing with the default */
blocklist_default_start:
.long 2 /* this is the sector start parameter, in logical
sectors from the start of the disk, sector 0 */
blocklist_default_len:
/* this is the number of sectors to read */
#ifdef STAGE1_5
.word 0 /* the command "install" will fill this up */
#else
.word (STAGE2_SIZE + 511) >> 9
#endif
blocklist_default_seg:
#ifdef STAGE1_5
.word 0x220
#else
.word 0x820 /* this is the segment of the starting address
to load the data into */
#endif
firstlist: /* this label has to be after the list data!!! */
