说明
- 本学习系列代码几乎完全摘自：asmtutor.com，如果英文可以的（也可以用谷歌浏览器翻译看），可以直接看asmtutor.com上的教程
- 本学习系列目录地址：https://www.cnblogs.com/whuwzp/p/nasm_contents.html
- 系统环境搭建：（我用的是ubuntu18.04.4 server，安装gcc、g++）
sudo apt install nasm
sudo apt install gcc-multilib
0. 概览
- 承前：上一节，我们调用sys_write时第三个参数字符串长度是固定长度13，那么如果改变了字符串，我们就得改，很不方便，能不能自动计算长度呢？
- 启后：本节，自动计算字符串长度，并且改写成函数形式，调用自定义的strlen函数
1. 计算字符串长度
算法思路是：
- ebx和eax先同时赋值为msg的地址；
- eax递增（地址+1），比较[eax]==' '，如果不是0就继续递增eax，否则退出
- 计算eax-ebx，这样就是字符串的长度了，差值保存在eax中
以下代码摘自：https://asmtutor.com/#lesson3
; Hello World Program (Calculating string length)
; Compile with: nasm -f elf helloworld-len.asm
; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 helloworld-len.o -o helloworld-len
; Run with: ./helloworld-len
SECTION .data
msg db 'Hello, brave new world!', 0Ah ; we can modify this now without having to update anywhere else in the program
SECTION .text
global _start
_start:
mov ebx, msg ; move the address of our message string into EBX
mov eax, ebx ; move the address in EBX into EAX as well (Both now point to the same segment in memory)
nextchar:
cmp byte [eax], 0 ; compare the byte pointed to by EAX at this address against zero (Zero is an end of string delimiter)
jz finished ; jump (if the zero flagged has been set) to the point in the code labeled 'finished'
inc eax ; increment the address in EAX by one byte (if the zero flagged has NOT been set)
jmp nextchar ; jump to the point in the code labeled 'nextchar'
finished:
sub eax, ebx ; subtract the address in EBX from the address in EAX
; remember both registers started pointing to the same address (see line 15)
; but EAX has been incremented one byte for each character in the message string
; when you subtract one memory address from another of the same type
; the result is number of segments between them - in this case the number of bytes
mov edx, eax ; EAX now equals the number of bytes in our string
mov ecx, msg ; the rest of the code should be familiar now
mov ebx, 1
mov eax, 4
int 80h
mov ebx, 0
mov eax, 1
int 80h
编译、链接和运行方法：（其实代码中已经写了）
nasm -f elf helloworld-len.asm
ld -m elf_i386 helloworld-len.o -o helloworld-len
# Run with:
./helloworld-len
2. 将计算长度的功能写为函数
如果以后也要用到计算字符串长度就很麻烦，所以写成函数，提升代码重用率。
以下摘自：https://asmtutor.com/#lesson4
; Hello World Program (Subroutines)
; Compile with: nasm -f elf helloworld-len.asm
; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 helloworld-len.o -o helloworld-len
; Run with: ./helloworld-len
SECTION .data
msg db 'Hello, brave new world!', 0Ah
SECTION .text
global _start
_start:
mov eax, msg ; move the address of our message string into EAX
call strlen ; call our function to calculate the length of the string
mov edx, eax ; our function leaves the result in EAX
mov ecx, msg ; this is all the same as before
mov ebx, 1
mov eax, 4
int 80h
mov ebx, 0
mov eax, 1
int 80h
strlen: ; this is our first function declaration
push ebx ; push the value in EBX onto the stack to preserve it while we use EBX in this function
mov ebx, eax ; move the address in EAX into EBX (Both point to the same segment in memory)
nextchar: ; this is the same as lesson3
cmp byte [eax], 0
jz finished
inc eax
jmp nextchar
finished:
sub eax, ebx
pop ebx ; pop the value on the stack back into EBX
ret ; return to where the function was called
这一段意思很简单，不多说：
- 函数调用参数依次保存在eax, ebx, ecx, edx中
- 函数的返回值存放于eax，
call strlen之后的mov edx, eax，就是把计算结果（字符串长度eax赋值到edx中，这个edx是sys_write的第三个参数） - 注意：strlen中的
push ebx和pop ebx从功能上是没啥意义的，但是，后面的示例中将会大量使用push eax, pop eax, push ebx, pop ebx等，这样做是为了保存寄存器中的内容，因为strlen中mov ebx, eax把ebx中的内容修改了，当我们退出strlen函数后，ebx的值就不是原先的值了，这样可能会对调用strlen的函数造成影响。所以push ebx保存到栈上，然后退出函数前pop ebx还原，这样函数返回后，寄存器的状态还是和以前一样，没有影响（eax作为保存返回值的寄存器，内容会改变）