Linux0.11内核--启动引导代码分析bootsect.s

文章由LinuxBoy分享于2019-03-31 08:03:06热评（232）

Linux0.11内核--启动引导代码分析bootsect.s

Linux内核中的系统启动引导代码位于／boot目录下

但是，由于。Linus当时是在MINIX系统上开发Linux的，最初MINIX系统上还没有移植gas程序，因此Linus就使用了MINIX系统上的as86。

bootsect需要是16位的实模式程序。目前gas汇编器也支持16位的编译。所以现在也可以直接用as编译器直接编译。

下面是用at&t格式改写的bootsect.s代码

.code16
# rewrite with AT&T syntax by falcon <wuzhangjin@gmail.com> at 081012
#
# SYS_SIZE is the number of clicks (16 bytes) to be loaded.
# 0x3000 is 0x30000 bytes = 196kB, more than enough for current
# versions of linux
#
.equ SYSSIZE, 0x3000
#
# bootsect.s (C) 1991 Linus Torvalds
#
# bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves
# iself out of the way to address 0x90000, and jumps there.
#
# It then loads 'setup' directly after itself (0x90200), and the system
# at 0x10000, using BIOS interrupts.
#
# NOTE! currently system is at most 8*65536 bytes long. This should be no
# problem, even in the future. I want to keep it simple. This 512 kB
# kernel size should be enough, especially as this doesn't contain the
# buffer cache as in minix
#
# The loader has been made as simple as possible, and continuos
# read errors will result in a unbreakable loop. Reboot by hand. It
# loads pretty fast by getting whole sectors at a time whenever possible.
.global _start, begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text
.equ SETUPLEN, 4 # nr of setup-sectors
.equ BOOTSEG, 0x07c0 # original address of boot-sector
.equ INITSEG, 0x9000 # we move boot here - out of the way
.equ SETUPSEG, 0x9020 # setup starts here
.equ SYSSEG, 0x1000 # system loaded at 0x10000 (65536).
.equ ENDSEG, SYSSEG + SYSSIZE # where to stop loading
# ROOT_DEV: 0x000 - same type of floppy as boot.
# 0x301 - first partition on first drive etc
.equ ROOT_DEV, 0x301
ljmp $BOOTSEG, $_start
_start:
mov $BOOTSEG, %ax
mov %ax, %ds
mov $INITSEG, %ax
mov %ax, %es
mov $256, %cx
sub %si, %si
sub %di, %di
rep
movsw
ljmp $INITSEG, $go
go: mov %cs, %ax
mov %ax, %ds
mov %ax, %es
# put stack at 0x9ff00.
mov %ax, %ss
mov $0xFF00, %sp # arbitrary value >>512
# load the setup-sectors directly after the bootblock.
# Note that 'es' is already set up.
load_setup:
mov $0x0000, %dx # drive 0, head 0
mov $0x0002, %cx # sector 2, track 0
mov $0x0200, %bx # address = 512, in INITSEG
.equ AX, 0x0200+SETUPLEN
mov $AX, %ax # service 2, nr of sectors
int $0x13 # read it
jnc ok_load_setup # ok - continue
mov $0x0000, %dx
mov $0x0000, %ax # reset the diskette
int $0x13
jmp load_setup
ok_load_setup:
# Get disk drive parameters, specifically nr of sectors/track
mov $0x00, %dl
mov $0x0800, %ax # AH=8 is get drive parameters
int $0x13
mov $0x00, %ch
#seg cs
mov %cx, %cs:sectors+0 # %cs means sectors is in %cs
mov $INITSEG, %ax
mov %ax, %es
# Print some inane message
mov $0x03, %ah # read cursor pos
xor %bh, %bh
int $0x10
mov $38, %cx
mov $0x0007, %bx # page 0, attribute 7 (normal)
#lea msg1, %bp
mov $msg1, %bp
mov $0x1301, %ax # write string, move cursor
int $0x10
# ok, we've written the message, now
# we want to load the system (at 0x10000)
mov $SYSSEG, %ax
mov %ax, %es # segment of 0x010000
call read_it
call kill_motor
# After that we check which root-device to use. If the device is
# defined (#= 0), nothing is done and the given device is used.
# Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending
# on the number of sectors that the BIOS reports currently.
#seg cs
mov %cs:root_dev+0, %ax
cmp $0, %ax
jne root_defined
#seg cs
mov %cs:sectors+0, %bx
mov $0x0208, %ax # /dev/ps0 - 1.2Mb
cmp $15, %bx
je root_defined
mov $0x021c, %ax # /dev/PS0 - 1.44Mb
cmp $18, %bx
je root_defined
undef_root:
jmp undef_root
root_defined:
#seg cs
mov %ax, %cs:root_dev+0
# after that (everyting loaded), we jump to
# the setup-routine loaded directly after
# the bootblock:
ljmp $SETUPSEG, $0 #jump to 0x9020:0000(setup.s 程序的开始处)
# This routine loads the system at address 0x10000, making sure
# no 64kB boundaries are crossed. We try to load it as fast as
# possible, loading whole tracks whenever we can.
#
# in: es - starting address segment (normally 0x1000)
#
sread: .word 1+ SETUPLEN # sectors read of current track
head: .word 0 # current head
track: .word 0 # current track
read_it:
mov %es, %ax
test $0x0fff, %ax
die: jne die # es must be at 64kB boundary
xor %bx, %bx # bx is starting address within segment
rp_read:
mov %es, %ax
cmp $ENDSEG, %ax # have we loaded all yet?
jb ok1_read
ret
ok1_read:
#seg cs
mov %cs:sectors+0, %ax
sub sread, %ax
mov %ax, %cx
shl $9, %cx
add %bx, %cx
jnc ok2_read
je ok2_read
xor %ax, %ax
sub %bx, %ax
shr $9, %ax
ok2_read:
call read_track
mov %ax, %cx
add sread, %ax
#seg cs
cmp %cs:sectors+0, %ax
jne ok3_read
mov $1, %ax
sub head, %ax
jne ok4_read
incw track
ok4_read:
mov %ax, head
xor %ax, %ax
ok3_read:
mov %ax, sread
shl $9, %cx
add %cx, %bx
jnc rp_read
mov %es, %ax
add $0x1000, %ax
mov %ax, %es
xor %bx, %bx
jmp rp_read
read_track:
push %ax
push %bx
push %cx
push %dx
mov track, %dx
mov sread, %cx
inc %cx
mov %dl, %ch
mov head, %dx
mov %dl, %dh
mov $0, %dl
and $0x0100, %dx
mov $2, %ah
int $0x13
jc bad_rt
pop %dx
pop %cx
pop %bx
pop %ax
ret
bad_rt: mov $0, %ax
mov $0, %dx
int $0x13
pop %dx
pop %cx
pop %bx
pop %ax
jmp read_track
#/*
# * This procedure turns off the floppy drive motor, so
# * that we enter the kernel in a known state, and
# * don't have to worry about it later.
# */
kill_motor:
push %dx
mov $0x3f2, %dx
mov $0, %al
outsb
pop %dx
ret
sectors:
.word 0
msg1:
.byte 13,10
.ascii "Loading system ... compiled by yan!"
.byte 13,10,13,10
.org 508
root_dev:
.word ROOT_DEV
boot_flag:
.word 0xAA55
.text
endtext:
.data
enddata:
.bss
endbss:

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