01. 人工反编译 - bootsect.s

人造人 · 发表于 2020-5-4 17:20:59

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最近在学习linux 0.11源代码，bootsect.s 文件中有一个 read_it 函数，用来读取软盘上的system模块
这个函数是这个文件中最复杂的一个，相信不少同学被这个函数劝退，我也被劝退n次
这一次我使用人工反编译，把汇编语言翻译成语义上等效的C语言代码
这一次我终于理解了这个函数的逻辑，把自己的翻译结果发上来，希望可以帮到更多的同学，^_^

这个代码可以编译运行，如果有不理解的地方，还可以调试运行这个程序

#define SETUPLEN 4
#define SYSSIZE 0x3000
#define SYSSEG 0x1000
#define ENDSEG (SYSSEG + SYSSIZE)
const int sectors = 18;
int sread = 1 + SETUPLEN;
int head = 0;
int track = 0;
void read_track(int count) {
(void)count;
}
void read_it(int es) {
while(es & 0x0fff)
;
int offset = 0;
while(1) {
if(es >= ENDSEG)
return;
int count = sectors - sread;
if(count * 512 + offset > 0x10000)
count = (0x10000 - offset) / 512;
read_track(count);
int sum = sread + count;
if(sum == sectors) {
if(head == 1) {
head = 0; ++track;
}
else ++head;
sum = 0;
}
sread = sum;
offset += count * 512;
if(offset < 0x10000)
continue;
es += 0x1000;
offset = 0;
}
}
int main(void) {
read_it(SYSSEG);
return 0;
}

复制代码

人造人 · 发表于 2020-5-4 17:22:22

本帖最后由人造人于 2020-5-4 17:34 编辑

把 bootsect.s 也贴上来吧

!
! 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
!
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.
.globl begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text
SETUPLEN = 4 ! nr of setup-sectors
BOOTSEG = 0x07c0 ! original address of boot-sector
INITSEG = 0x9000 ! we move boot here - out of the way
SETUPSEG = 0x9020 ! setup starts here
SYSSEG = 0x1000 ! system loaded at 0x10000 (65536).
ENDSEG = SYSSEG + SYSSIZE ! where to stop loading
! ROOT_DEV: 0x000 - same type of floppy as boot.
! 0x301 - first partition on first drive etc
ROOT_DEV = 0x306
entry start
start:
mov ax,#BOOTSEG
mov ds,ax
mov ax,#INITSEG
mov es,ax
mov cx,#256
sub si,si
sub di,di
rep
movw
jmpi go,INITSEG
go: mov ax,cs
mov ds,ax
mov es,ax
! put stack at 0x9ff00.
mov ss,ax
mov sp,#0xFF00 ! arbitrary value >>512
! load the setup-sectors directly after the bootblock.
! Note that 'es' is already set up.
load_setup:
mov dx,#0x0000 ! drive 0, head 0
mov cx,#0x0002 ! sector 2, track 0
mov bx,#0x0200 ! address = 512, in INITSEG
mov ax,#0x0200+SETUPLEN ! service 2, nr of sectors
int 0x13 ! read it
jnc ok_load_setup ! ok - continue
mov dx,#0x0000
mov ax,#0x0000 ! reset the diskette
int 0x13
j load_setup
ok_load_setup:
! Get disk drive parameters, specifically nr of sectors/track
mov dl,#0x00
mov ax,#0x0800 ! AH=8 is get drive parameters
int 0x13
mov ch,#0x00
seg cs
mov sectors,cx
mov ax,#INITSEG
mov es,ax
! Print some inane message
mov ah,#0x03 ! read cursor pos
xor bh,bh
int 0x10
mov cx,#24
mov bx,#0x0007 ! page 0, attribute 7 (normal)
mov bp,#msg1
mov ax,#0x1301 ! write string, move cursor
int 0x10
! ok, we've written the message, now
! we want to load the system (at 0x10000)
mov ax,#SYSSEG
mov es,ax ! 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 ax,root_dev
cmp ax,#0
jne root_defined
seg cs
mov bx,sectors
mov ax,#0x0208 ! /dev/ps0 - 1.2Mb
cmp bx,#15
je root_defined
mov ax,#0x021c ! /dev/PS0 - 1.44Mb
cmp bx,#18
je root_defined
undef_root:
jmp undef_root
root_defined:
seg cs
mov root_dev,ax
! after that (everyting loaded), we jump to
! the setup-routine loaded directly after
! the bootblock:
jmpi 0,SETUPSEG
! 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 ax,es
test ax,#0x0fff
die: jne die ! es must be at 64kB boundary
xor bx,bx ! bx is starting address within segment
rp_read:
mov ax,es
cmp ax,#ENDSEG ! have we loaded all yet?
jb ok1_read
ret
ok1_read:
seg cs
mov ax,sectors
sub ax,sread
mov cx,ax
shl cx,#9
add cx,bx
jnc ok2_read
je ok2_read
xor ax,ax
sub ax,bx
shr ax,#9
ok2_read:
call read_track
mov cx,ax
add ax,sread
seg cs
cmp ax,sectors
jne ok3_read
mov ax,#1
sub ax,head
jne ok4_read
inc track
ok4_read:
mov head,ax
xor ax,ax
ok3_read:
mov sread,ax
shl cx,#9
add bx,cx
jnc rp_read
mov ax,es
add ax,#0x1000
mov es,ax
xor bx,bx
jmp rp_read
read_track:
push ax
push bx
push cx
push dx
mov dx,track
mov cx,sread
inc cx
mov ch,dl
mov dx,head
mov dh,dl
mov dl,#0
and dx,#0x0100
mov ah,#2
int 0x13
jc bad_rt
pop dx
pop cx
pop bx
pop ax
ret
bad_rt: mov ax,#0
mov dx,#0
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 dx,#0x3f2
mov al,#0
outb
pop dx
ret
sectors:
.word 0
msg1:
.byte 13,10
.ascii "Loading system ..."
.byte 13,10,13,10
.org 508
root_dev:
.word ROOT_DEV
boot_flag:
.word 0xAA55
.text
endtext:
.data
enddata:
.bss
endbss:

复制代码

老八秘制 · 发表于 2020-5-4 17:24:10

初学C语言表示瑟瑟发抖

Hello. · 发表于 2020-5-4 17:25:59

十分有用！！

人造人 · 发表于 2020-5-4 17:27:04

老八秘制发表于 2020-5-4 17:24
初学C语言表示瑟瑟发抖

^_^

乘号 · 发表于 2020-5-4 17:50:38

没学c语言表示瑟瑟发抖

即将被Hello.攻破电脑表示瑟瑟发抖

人造人 · 发表于 2020-5-4 17:54:05

乘号发表于 2020-5-4 17:50
没学c语言表示瑟瑟发抖

即将被Hello.攻破电脑表示瑟瑟发抖

C语言不难，真的，^_^

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