#
以上的Coyright就不用翻译了。
# $FreeBSD: src/sys/boot/i386/boot0/boot0.s,v 1.27 2003/11/20 20:28:18 jhb Exp $
以上供版本管理软件使用
# Copyright (c) 1998 Robert NordIEr
# All rights reserved.
#
# Redistribution and use in source and binary forms are freely
# permitted provided that the above copyright notice and this
# paragraph and the following disclaimer are duplicated in all
# such forms.
#
# This software is provided "AS IS" and without any express or
# implied warranties, including, without limitation, the implied
# warranties of merchantability and fitness for a particular
# purpose.
## A 512-byte boot manager.
以上是定义相关的参数值,例如“.set NHRDRV,0x475类似于C语言中的“#define NHRDRV 0x475
.set NHRDRV,0x475 # Number of hard drives
.set ORIGIN,0x600 # Execution address
.set FAKE,0x800 # Partition entry
.set LOAD,0x7c00 # Load address
.set PRT_OFF,0x1be # Partition table
.set TBL0SZ,0x3 # Table 0 size
.set TBL1SZ,0xb # Table 1 size
.set MAGIC,0xaa55 # Magic: bootable
.set B0MAGIC,0xbb66 # Identification
.set KEY_ENTER,0x1c # Enter key scan code
.set KEY_F1,0x3b # F1 key scan code
.set KEY_1,0x02 # #1 key scan code
#
# Addresses in the sector of embedded data values.
# Accessed with negative offsets from the end of the relocated sector (%ebp).
#
.set _NXTDRV,-0x48 # Next drive
.set _OPT,-0x47 # Default option
.set _SETDRV,-0x46 # Drive to force
.set _FLAGS,-0x45 # Flags
.set _TICKS,-0x44 # Timeout ticks
.set _FAKE,0x0 # Fake partition entry
.set _MNUOPT,0xc # Menu options .globl start # Entry point
以上代码:
1)首先使用“cld指令清除方向标志,使得以下的进行“rep操作时SI和DI的值递增。
2)使ax清零,并使除代码段cs外的另外两个数据段寄存器es、ds和堆栈段ss清零。当然,此时cs由于reset或初始上电已经为零了。
3)BIOS已经把引导扇区的512字节的内容读入到了0:0x7c00处,movw $LOAD,%sp 使得堆栈指针指向扇区代码(或曰本段代码 0:0x7c00)的顶部。虽然堆栈向下生长可能会影响代码的内容,但下面我们马上就把位于0:7c00处代码移到其他地方去执行。
.code16 # This runs in real mode
#
# Initialise segments and registers to known values.
# segments start at 0.
# The stack is immediately below the address we were loaded to.
#
start:
cld # String ops inc
xorw %ax,%ax # Zero
movw %ax,%es # Address
movw %ax,%ds # data
movw %ax,%ss # Set up
movw $LOAD,%sp # stack#
把位于0:7c00处的代码搬移到0:0x600处。注意,此时由于代码连接的重定向,$start=0x600。
# Copy this code to the address it was linked for
#
movw %sp,%si # Source
movw $start,%di # Destination
movw $0x100,%cx # Word count
rep # Relocate
movsw # code#
通过以上一段代码的执行,本代码已被搬移到0:0x600处,此时si=di=0x600+0x100,以上代码把di的值保存到bp,bp此时指向本程序搬移后的未用的空间的首部,且把此bp所指的16字节空间清零。以上过程如下图所示:
┏>0:0x600 ┏━━━━━┓
┃ ┃ ┃
┃ ┃ 搬 ┃
┃ ┃ 移 ┃
┃ ┃ 之 ┃
┃ ┃ 后 ┃
┃ ┃ 的 ┃
┃ ┃ 代 ┃
┃ ┃ 码 ┃
┃ ┃ ┃
┃ 0:0x7ff ┣━━━━━┫
┃ ┃ 0 ┃<-bp指向这里(0:0x800),以此开始的16字节被清零。
┃ ┣━━━━━┫以下所称的fake partition entry就是指这里。
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┣━━━━━┫
┃ ┃ 0 ┃
┃ ┗━━━━━┛
0:0x7c00 ┏━━━━━┓ ┛
┃ ┃
┃ 搬 ┃
┃ 移 ┃
┃ 之 ┃
┃ 前 ┃
┃ 的 ┃
┃ 代 ┃
┃ 码 ┃
┃ ┃
0:0x7dff ┗━━━━━┛
图(二)
# Set address for variable space beyond code, and clear it.
# Notice that this is also used to point to the values embedded in the block,
# by using negative offsets.
movw %di,%bp # Address variables
movb $0x8,%cl # Words to clear
rep # Zero
stosw # them#
把以上清零的16字节的第二个字节置为1,表示我们已经读取了一个分区。然后跳转到搬移之后的新代码的main处执行。
# Relocate to the new copy of the code.
#
incb -0xe(%di) # Sector number
jmp main-LOAD+ORIGIN # To relocated code#
上面说过,BIOS把磁盘的引导扇区读入到内存之后,其dl的内容表示启动设备,但我们安装好FreeBSD之后,我们可以改变此引导扇区的内容,其中的一个改变就是可以使我们可以“手动指定我们实际安装FreeBSD的分区,如果我们希望指定FreeBSD所在的boot分区,那么我们在bp-0x45处的位置(即_FLAGS(%bp)处)的bit 0x20置1,那么上面的“movb _SETDRV(%bp),%dl一句中movb_SETDRV(%bp),%dl(即bp-0x46)即指向我们“手动指定FreeBSD所在分区代码,例如,IDE的C、D盘(严格来说是第一个物理磁盘的第一个和第二个分区)的代码分别为 0x80和0x81。如果没有“手动指定启动分区,那么,我们缺省使用机器当前启动的分区,上面说过,机器当前启动的分区代码放在dl中。因为FreeBSD Boot Manager 不可能安装到软盘(如果从软盘启动则dl为0),所以,使用testb %dl,%dl来判断驱动器代码是否合法(volid)。
有关_FLAGS(%bp)中其他bit位表示的意义,在随后的代码分析中慢慢给你道来。
# Check what flags were loaded with us, specifically, Use a predefined Drive.
# If what the bios gives us is bad, use the '0' in the block instead, as well.
#
main:
testb $0x20,_FLAGS(%bp) # Set number drive?
jnz main.1 # Yes
testb %dl,%dl # Drive number valid?
js main.2 # Possibly (0x80 set)
main.1:
movb _SETDRV(%bp),%dl # Drive number to use#
以上第一句把FreeBSD启动分区的代码保存到_FAKE(%bp)(bp-0)处,也就是说,上图(二)的bp处保存的是FreeBSD启动分区的代码(_FAKE=0)。
“callw putn一句在屏幕上打印“回车和“换行,“pushw %dx一句把启动分区的值压入堆栈。
# Whatever we decided to use, now store it into the fake
# partition entry that lives in the data space above us.
#
main.2:
movb %dl,_FAKE(%bp) # Save drive number
callw putn # To new line
pushw %dx # Save drive number#
以上代码首先把%bx指向分区表partbl的的第四个字节,这里存放的是分区类型,如82表示Linux Native分区83表示Linux Swap 分区,有关分区表的细节请详见本文的尾部。然后dx清零,此后,dx将作为遍历磁盘分区的列举代号使用。启动分区代码dl的原来的值在上面已经压入了堆栈保存。
# Start out with a pointer to the 4th byte of the first table entry
# so that after 4 iterations it's beyond the end of the sector.
# and beyond a 256 byte boundary and has overflowed 8 bits (see next comment).
# (remember that the table starts 2 bytes earlier than you would expect
# as the bootable flag is after it in the block)
#
movw $(partbl+0x4),%bx # Partition table (+4)
xorw %dx,%dx # Item number#
上面首先使得第一个分区的活动标志为0,标志它不为活动标志,因为ch的值为0。至于第二句“btw %dx,_FLAGS(%bp)中的_FLAGS(%bp)是上面我们说到的“手动指定我们实际安装FreeBSD的分区代码。其中的bit 0x20我们在上面已经提到过。_FLAGS(%bp)中的其他位表示是否我们需要检查相应的磁盘分区。缺省情况下,我们需要检查所有的磁盘分区。检查磁盘分区看是否有可以启动的磁盘分区,例如,可能磁盘上的某个分区为WindowsXP或者是Linux等。如果我们没有改变在磁盘上该处的值,则相应的bit位的值为0,表示所有的分区都要检查(因为此时_FLAGS(%bp)中的值为0),否则,只针对FLAGS(%bp)中相应的bit位未被设置为1的分区进行检查。
大家知道,FreeBSD Manager启动时可能出现以下的提示:
# Loop around on the partition table, printing values until we
# pass a 256 byte boundary. The end of loop test is at main.5.
#
main.3:
movb %ch,-0x4(%bx) # Zero active flag (ch == 0)
btw %dx,_FLAGS(%bp) # Entry enabled?
jnc main.5 # NoF1 FreeBSD
其中,上面的提示中出现了令人讨厌的“??,为了避免出现“??的提示,我们可以设置相应的第一分区和第四分区不检查,就需要正确设置_FLAGS(%bp)中的bit位。设置好后,屏幕可能出现以下的提示:
F2 ??
F3 BSD
F4 ??
Default F1F1 FreeBSD
我们从上面已经知道起始(%bx)指向的是MBR中分区信息1(16字节)的位置(见图(三)),以上代码在“忽略的分区类型$tables中查找看是否本分区是不可启动的或者不合法的分区。不可启动的或者不合法的分区类型有3($TBL0SZ=3)个,它们是“0x0, 0x5, 0xf,见下面的$tables处。如果是不可启动的或者不合法的分区类型则跳转到main.5,进行下一轮循环。
F2 BSD
Default F1
#
# If any of the entries in the table are
# the same as the 'type' in the slice table entry,
# then this is an empty or non bootable partition. Skip it.
#
movb (%bx),%al # Load type
movw $tables,%di # Lookup tables
movb $TBL0SZ,%cl # Number of entries
repne # Exclude
scasb # partition?
je main.5 # Yes#
上面检查看所检查的分区类型是否为我们知道的分区类型,知道的分区类型有11($TBL1SZ=0xb)个,它们是:“0x1, 0x4, 0x6, 0xb, 0xc, 0xe, 0x83,0x9f, 0xa5, 0xa6, 0xa9,见下面的$tables处。如果不是以上的类型,则跳转到main.4。那么,(%di)所指的字串是“??,如果分区类型是“0x1, 0x4, 0x6, 0xb, 0xc, 0xe, 0x83,0x9f, 0xa5, 0xa6, 0xa9之一,则(%di)所指的字串是“Dos、“Linux、“FreeBSD或“BSD等。见下面的“os_misc、“os_dos、“os_linux、“os_freebsd、“os_bsd等标记。
callw putx调用putx函数,在屏幕上打印:“Fx XXX。其中XXX为DOS、“Linux、“FreeBSD或“BSD等。
# Now scan the table of known types
#
movb $TBL1SZ,%cl # Number of entries
repne # Known
scasb # type?
jne main.4 # No
#
# If it matches get the matching element in the
# next array. if it doesn't, we are already
# pointing at its first element which points to a "?".
#
addw $TBL1SZ,%di # Adjust
main.4:
movb (%di),%cl # Partition
addw %cx,%di # description
callw putx # Display itmain.5:
遍历磁盘分区的举代号dx加1,重复下一轮循环查找。bl加上0x10(0x10=16)表示寻址到下一个分区信息(加16字节)入口。循环直到255字节边界。
incw %dx # Next item
addb $0x10,%bl # Next entry
jnc main.3 # Till done#
“popw %ax把上面压入堆栈的bx(当前的启动扇区)值弹出到ax中。例如,如果计算机是从软盘启动的则dl=0,若是从IDE的C、D盘启动的则dl分别为 0x80和0x81。然而,FreeBSD的Boot Manerger不能够安装到软盘上,所以,dl只能为0x80、0x81,0x82...等。在计算机的BIOS地址0:0x475处存放的是计算机的硬盘的数目,“subb $0x80-0x1,%al一句等于“sub$0x79,%al,例如,即当前驱动器如果是C盘,则al的值是ox80-0x79=1,然后再与计算机的硬盘的数目比较,如果当前所在硬盘不是最后一个硬盘,则直接跳转到main.6。如果当前所在硬盘是最后一个硬盘,则继续执行。
# Passed a 256 byte boundary..
# table is finished.
# Add one to the drive number and check it is valid,
#
popw %ax # Drive number
subb $0x80-0x1,%al # Does next
cmpb NHRDRV,%al # drive exist? (from BIOS?)
jb main.6 # Yes# If not then if there is only one drive,
如果只有一个硬盘,则直接跳转到main.7,这样,本计算机只有一个硬盘,所以不用显示其他
磁盘相关的提示。
# Don't display drive as an option.
#
decw %ax # Already drive 0?
jz main.7 # Yes# If it was illegal or we cycled through them,
下面的内容表示多于一个磁盘的情况。此时“al清0,与磁盘列举相关。
# then go back to drive 0.
#
xorb %al,%al # Drive 0#
首先,在_NXTDR(%bp)处置入“0字符高位置1的字符,以代表第二个驱动器,然后在屏幕上显示“Fx Drive,表示更换另外的磁盘启动。注意,在调用putx之前,di中保存的是下面字串“Drive 的首地址。dl中存放的是当前遍历的到的可启动的或者合法的分区类型递增序数,al与dl是不同的,al是ASCII码,dl是“Fx中的x值。
# Whatever drive we selected, make it an ascii digit and save it back
# to the "next drive" location in the loaded block in case we
# want to save it for next time.
# This also is part of the printed drive string so add 0x80 to indicate
# end of string.
#
main.6:
addb $'0'|0x80,%al # Save next
movb %al,_NXTDRV(%bp) # drive number
movw $drive,%di # Display
callw putx # item#
上面的代码首先在屏幕上打印出字符串“Default: ,缺省启动的磁盘号放在“_OPT(%bp)中,这里有个小小的技巧,在执行“decw %si和“callw putkey两句后屏幕会显示“Fx,x是_OPT(%bp)的ASCII。然后取得当前的tickes放到%di中,等待用户按键超时的时间从_TICKS(%bp)中取出,加到当前的tickes即是最后超时时间到的tickes。
# Now that we've printed the drive (if we needed to), display a prompt.
# Get ready for the input byt noting the time.
#
main.7:
movw $prompt,%si # Display
callw putstr # prompt
movb _OPT(%bp),%dl # Display
decw %si # default
callw putkey # key
xorb %ah,%ah # BIOS: Get
int $0x1a # system time
movw %dx,%di # Ticks when
addw _TICKS(%bp),%di # timeout#
等待用户按下“Fx键,同时检查当前等待是否超时,如果有用户按键则跳转到main.11,
如果超时时间不到则继续等待。
# Busy loop, looking for keystrokes but
# keeping one eye on the time.
#
main.8:
movb $0x1,%ah # BIOS: Check
int $0x16 # for keypress
jnz main.11 # Have one
xorb %ah,%ah # BIOS: Get
int $0x1a # system time
cmpw %di,%dx # Timeout?
jb main.8 # No#
超时时间到,此时表示用户使用缺省分区启动,把缺省的启动分区号置入al中,然后跳转
到main.12。
# If timed out or defaulting, come here.
#
main.9:
movb _OPT(%bp),%al # Load default
jmp main.12 # Join common code#
用户输入错误,只是响铃提示,其他什么也不发生。
# User's last try was bad, beep in displeasure.
# Since nothing was printed, just continue on as if the user
# hadn't done anything. This gives the effect of the user getting a beep
# for all bad keystrokes but no action until either the timeout
# occurs or the user hits a good key.
#
main.10:
movb $0x7,%al # Signal
callw putchr # error#
用户按下了一个键,把键值扫描码放到al中。
# Get the keystroke.
#
main.11:
xorb %ah,%ah # BIOS: Get
int $0x16 # keypress
movb %ah,%al # Scan code#
如果用户按下“Enter键,和超时等同处理,这样,就启动缺省的boot分区。
# If it's CR act as if timed out.
#
cmpb $KEY_ENTER,%al # Enter pressed?
je main.9 # Yes#
如果是除“Enter键外其他的键,则检查是不是F1...F5键,如果不是,表示输入不合法,跳回到main.10处理。
# Otherwise check if legal
# If not ask again.
#
subb $KEY_F1,%al # Less F1 scan code
cmpb $0x4,%al # F1..F5?
jna main.12 # Yes
subb $(KEY_1 - KEY_F1),%al # Less #1 scan code
cmpb $0x4,%al # #1..#5?
ja main.10 # No#
如果是F1...F5键,则检查是否在我们提示的范围内,其中,_MNUOPT(%bp)的相应bit位为1,表示是一个合法的选项,如果不是,跳回到 main.10处理。
# We have a selection.
# but if it's a bad selection go back to complain.
# The bits in MNUOPT were set when the options were printed.
# Anything not printed is not an option.
#
main.12:
cbtw # Option
btw %ax,_MNUOPT(%bp) # enabled?
jnc main.10 # No#
把我们按下的F1...F5键保存到_OPT(%bp)位置。
# Save the info in the original tables
# for rewriting to the disk.
#
movb %al,_OPT(%bp) # Save option movw $FAKE,%si # Partition for write
把原来的启动分区代码取回到dl中。
movb (%si),%dl # Drive number movw %si,%bx # Partition for read
如果我们按下的是F5键则直接跳转到main.13处理。
cmpb $0x4,%al # F5 pressed?
pushf # Save
je main.13 # Yes shlb $0x4,%al # Point to
上面,我们从按键Fx选择中得到图(三)中的我们选择的四个分区信息中的某一分区信息,上面计算出的bx为我们选择的分区信息的首地址,我们把此选择到的分区信息的第一个个字节置为0x80表示它是当前的活动分区。
addw $partbl,%ax # selected
xchgw %bx,%ax # partition
movb $0x80,(%bx) # Flag active#
检查回写标志_FLAGS(%bp)的bit位0x40为,如果设置的是可回写,则把当前选择到的boot分区作为下次缺省的启动分区。
# If not asked to do a write-back (flags 0x40) don't do one.
#
main.13:
pushw %bx # Save
testb $0x40,_FLAGS(%bp) # No updates?
jnz main.14 # Yes
movw $start,%bx # Data to write
movb $0x3,%ah # Write sector
callw intx13 # to diskmain.14:
恢复上面保存的si和标志寄存器的内容。如果不是按键F5,则直接跳转到main.15去执行。
popw %si # Restore
popf # Restore
#
# If going to next drive, replace drive with selected one.
# Remember to un-ascii it. Hey 0x80 is already set, cool!
#
jne main.15 # If not F5 movb _NXTDRV(%bp),%dl # Next drive
否则的话,我们选择下一个驱动器作为启动盘。
subb $'0',%dl # number#
把我们上面选择到的分区读到0x7c00处,就象我们刚刚才加电启动一样,只是活动分区改变了而已。如果发生读错误则直接跳转到main.10。使用户重新选择启动分区。
cmpw $MAGIC,0x1fe(%bx) # Bootable?
jne main.10 # No
判断引导扇区的最后两个字节是否是“0x55AA,如果不是,则表示此扇区是不可引导的,或不合法的引导扇区则直接跳转到main.10。使用户重新选择启动分区。
pushw %si # Save
movw $crlf,%si # Leave some
callw puts # space
popw %si # Restore
打印“回车和“换行。
jmp *%bx # Invoke bootstrap
跳转到我们选择的引导扇区去执行。整个Boot Manager代码到此执行完毕。
# load selected bootsector to the LOAD location in RAM.
# If it fails to read or isn't marked bootable, treat it
# as a bad selection.
# XXX what does %si carry?
#
main.15:
movw $LOAD,%bx # Address for read
movb $0x2,%ah # Read sector
callw intx13 # from disk
jc main.10 # If error#
“putkey函数在屏幕上打印“F1、“F2或“F3等。如果dl为0则打印“F1,
如果dl为1则打印“F2,如果dl为3则打印“F3。和调用“putstr在屏幕上打印
es:si指向的以最高位置1为结束字符的字符串。
# Display routines
#
putkey:
movb $'F',%al # Display
callw putchr # 'F'
movb $'1',%al # Prepare
addb %dl,%al # digit
jmp putstr.1 # Display the rest#
putx: #首先,把_MNUOPT(%bp)的第dx位(bit)置1,表示此菜单选项被显示。然后在屏幕上打印空格和es:di指向的以最高位置1为结束字符的字符串。
# Display the option and note that it is a valid option.
# That last point is a bit tricky..
# btsw %dx,_MNUOPT(%bp) # Enable menu option
puts: #调用“putstr在屏幕上打印es:si指向的以最高位置1为结束字符的字符串。
callw putstr # Display string
putn: #“putn打印“回车/换行后在屏幕上打印es:si指向的以最高位置1为结束字符的字符串。
movw $item,%si # Display
callw putkey # key
movw %di,%si # Display the rest movw $crlf,%si # To next line
“putchr在屏幕上显示“al中的字符。
putstr: #“putstr在屏幕上打印es:si指向的以最高位置1为结束字符的字符串。
lodsb # Get byte
testb $0x80,%al # End of string?
jnz putstr.2 # Yes
putstr.1:
callw putchr # Display char
jmp putstr # Continue
putstr.2:
andb $~0x80,%al # Clear MSB
putchr:
pushw %bx # Save
movw $0x7,%bx # Page:attribute
movb $0xe,%ah # BIOS: Display
int $0x10 # character
popw %bx # Restore
retw # To caller# One-sector disk I/O routine
intx13:
movb 0x1(%si),%dh # Load head
movw 0x2(%si),%cx # Load cylinder:sector
movb $0x1,%al # Sector count
pushw %si # Save
movw %sp,%di # Save
testb $0x80,_FLAGS(%bp) # Use packet interface?
jz intx13.1 # No
pushl $0x0 # Set the
pushl 0x8(%si) # LBA address
pushw %es # Set the transfer
pushw %bx # buffer address
push $0x1 # Block count
push $0x10 # Packet size
movw %sp,%si # Packet pointer
decw %ax # Verify off
orb $0x40,%ah # Use disk packet
intx13.1:
int $0x13 # BIOS: Disk I/O
movw %di,%sp # Restore
popw %si # Restore
retw # To caller
# Menu strings
item:
.ascii " "; .byte ' '|0x80
prompt:
.ascii "nDefault:".byte ' '|0x80
crlf:
.ascii "r"; .byte 'n'|0x80
# Partition type tables
tables:
#
# These entries identify invalid or NON BOOT types and partitions.
#
.byte 0x0, 0x5, 0xf
#
# These values indicate bootable types we know the names of
#
.byte 0x1, 0x4, 0x6, 0xb, 0xc, 0xe, 0x83
.byte 0x9f, 0xa5, 0xa6, 0xa9
#
# These are offsets that match the known names above and point to the strings
# that will be printed.
#
.byte os_misc-. # Unknown
.byte os_dos-. # DOS
.byte os_dos-. # DOS
.byte os_dos-. # DOS
.byte os_dos-. # Windows
.byte os_dos-. # Windows
.byte os_dos-. # Windows
.byte os_linux-. # Linux
.byte os_bsd-. # BSD/OS
.byte os_freebsd-. # FreeBSD
.byte o
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