指示把代码划分成若干个段（Section）,被操作系统加载执行时，每个段被加载到不同的地址
保存程序的数据,可读可写，相当于C程序的全局变量
保存代码,是只读和可执行的
传送指令
比较指令
条件跳转指令(jump if equal).判断eflags寄存器的ZF位
自增指令
条件跳转指令(jump if less than or equal)
跳转指令
软中断指令
汇编器将助记符变成机器指令，生成目标文件.
>>> as hello.s -o hello.o
链接器将目标文件链接成可执行文件
>>> ld hello.o -o hello
读取ELF文件
>>> readelf -a hello.o
>>> readelf -a hello
#PURPOSE: Simple program that exits and returns a
# status code back to the Linux kernel
#
#INPUT: none
#
#OUTPUT: returns a status code. This can be viewed
# by typing
#
# echo $?
#
# after running the program
#
#VARIABLES:
# %eax holds the system call number
# %ebx holds the return status
#
.section .data
.section .text
.globl _start
_start:
movl $1, %eax # this is the linux kernel command
# number (system call) for exiting
# a program
movl $4, %ebx # this is the status number we will
# return to the operating system.
# Change this around and it will
# return different things to
# echo $?
int $0x80 # this wakes up the kernel to run
# the exit command
#PURPOSE: This program finds the maximum number of a
# set of data items.
#
#VARIABLES: The registers have the following uses:
#
# %edi - Holds the index of the data item being examined
# %ebx - Largest data item found
# %eax - Current data item
#
# The following memory locations are used:
#
# data_items - contains the item data. A 0 is used
# to terminate the data
#
.section .data
data_items: #These are the data items
.long 3,67,34,222,45,75,54,34,44,33,22,11,66,0
.section .text
.globl _start
_start:
movl $0, %edi # move 0 into the index register
movl data_items(,%edi,4), %eax # load the first byte of data
movl %eax, %ebx # since this is the first item, %eax is
# the biggest
start_loop: # start loop
cmpl $0, %eax # check to see if we've hit the end
je loop_exit
incl %edi # load next value
movl data_items(,%edi,4), %eax
cmpl %ebx, %eax # compare values
jle start_loop # jump to loop beginning if the new
# one isn't bigger
movl %eax, %ebx # move the value as the largest
jmp start_loop # jump to loop beginning
loop_exit:
# %ebx is the status code for the _exit system call
# and it already has the maximum number
movl $1, %eax #1 is the _exit() syscall
int $0x80
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 200 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 40 (bytes)
Number of section headers: 8
Section header string table index: 5
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00000000 000034 00002a 00 AX 0 0 4
[ 2] .rel.text REL 00000000 0002b0 000010 08 6 1 4
[ 3] .data PROGBITS 00000000 000060 000038 00 WA 0 0 4
[ 4] .bss NOBITS 00000000 000098 000000 00 WA 0 0 4
[ 5] .shstrtab STRTAB 00000000 000098 000030 00 0 0 1
[ 6] .symtab SYMTAB 00000000 000208 000080 10 7 7 4
[ 7] .strtab STRTAB 00000000 000288 000028 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
Relocation section '.rel.text' at offset 0x2b0 contains 2 entries:
Offset Info Type Sym.Value Sym. Name
00000008 00000201 R_386_32 00000000 .data
00000017 00000201 R_386_32 00000000 .data
There are no unwind sections in this file.
Symbol table '.symtab' contains 8 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 SECTION LOCAL DEFAULT 1
2: 00000000 0 SECTION LOCAL DEFAULT 3
3: 00000000 0 SECTION LOCAL DEFAULT 4
4: 00000000 0 NOTYPE LOCAL DEFAULT 3 data_items
5: 0000000e 0 NOTYPE LOCAL DEFAULT 1 start_loop
6: 00000023 0 NOTYPE LOCAL DEFAULT 1 loop_exit
7: 00000000 0 NOTYPE GLOBAL DEFAULT 1 _start
No version information found in this file.
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x8048074
Start of program headers: 52 (bytes into file)
Start of section headers: 256 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 2
Size of section headers: 40 (bytes)
Number of section headers: 6
Section header string table index: 3
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 08048074 000074 00002a 00 AX 0 0 4
[ 2] .data PROGBITS 080490a0 0000a0 000038 00 WA 0 0 4
[ 3] .shstrtab STRTAB 00000000 0000d8 000027 00 0 0 1
[ 4] .symtab SYMTAB 00000000 0001f0 0000a0 10 5 6 4
[ 5] .strtab STRTAB 00000000 000290 000040 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x08048000 0x08048000 0x0009e 0x0009e R E 0x1000
LOAD 0x0000a0 0x080490a0 0x080490a0 0x00038 0x00038 RW 0x1000
Section to Segment mapping:
Segment Sections...
00 .text
01 .data
There is no dynamic section in this file.
There are no relocations in this file.
There are no unwind sections in this file.
Symbol table '.symtab' contains 10 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 08048074 0 SECTION LOCAL DEFAULT 1
2: 080490a0 0 SECTION LOCAL DEFAULT 2
3: 080490a0 0 NOTYPE LOCAL DEFAULT 2 data_items
4: 08048082 0 NOTYPE LOCAL DEFAULT 1 start_loop
5: 08048097 0 NOTYPE LOCAL DEFAULT 1 loop_exit
6: 08048074 0 NOTYPE GLOBAL DEFAULT 1 _start
7: 080490d8 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
8: 080490d8 0 NOTYPE GLOBAL DEFAULT ABS _edata
9: 080490d8 0 NOTYPE GLOBAL DEFAULT ABS _end
No version information found in this file.
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 200 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 0 (bytes)