全称是Netwide Assembler的NASM, 是当下最为流行的开源汇编工具当中的一个, 它能够协助我们将写好的汇编代码转变为机器可以看懂的一串串二进制指令, 这件事情看上去复杂, 然而只要掌握了几个核心要点, 实际上并不困难, 下面从两个最关键的环节开始展开。汇编代码怎么转成机器码不少人初次碰到NASM之际会存有一个疑惑, 我撰写了汇编指令, 它究竟是怎样转变为0和1的。实际上NASM于这个进程当中所充当的乃是翻译官的角色。它逐行去读取你的汇编代码, 依据指令集表找寻到对应的操作码, 进而结合操作数计算出机器码。比如说, 你去写个mov eax, 1, NASM, 它会先从指令表当中去查到, mov eax, imm32所对应的操作码居然是B8, 接着, 它会把1转变成四字节小端序的机器码, 最后, 输出B8 01000000。这个过程看起来好像挺简单的, 然而, 里面是涉及到寻址模式、指令前缀、立即数编码等诸多细节的。常见的一个坑是立即数的大小写错了, 比如说你写mov al, 256, 然而AL只能存储一个字节, 256超出了其范围, NASM会直接报错, 这时你得改成mov ax, 256或者使用两字节指令, 这些细节在实际编码的时候经常会遇到。NASM使用中有哪些常见问题真实运用NASM之际, 最为容易出现问题的是段定义以及输出格式, NASM的默认输出形式是bin格式, 此格式即为纯粹的二进制格式, 适宜用于编写引导程序或者裸机代码, 然而要是期望生成Windows环境下的PE文件或者Linux环境下的ELF文件, 那想必就得借助-f win32或者-f elf32来指定格式: 。让人头疼的还有标签与跳转, 许多人写循环之时会直接写上jmp loop_start, 然而NASM对于标签大小写是敏感的, 并且要是你的标签位于别的段中, 若不加global声明将会链接失败, 我的建议即为, 写复杂项目之际尽量把标签全都声明成global, 免得后续排查链接错误。另有一项于实践当中极为有用处的技巧是此种操作 其乃是运用 “-l” 参数去生成列表 文件 举例来说 像 “nasm -f bin -l list.txt test.asm” 这般 此一列表文件会呈示出每一行汇编代码所对应的机器码以及地址偏移 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