为 mov 指令选择后缀 (l-b-w) [英] Chosing suffix (l-b-w) for mov instruction

问题描述

我是汇编新手.我正在阅读计算机系统程序员的观点.我不明白如何为 mov 指令选择后缀.我知道每个寄存器和位数.后缀使用由位数决定(32 位 l、16 位 w、8 位 b).很少有例子对前面的句子无效.例如 %esp 是 32 位寄存器，但对于 4. 步骤后缀 b 用于代替 l.请给出使用后缀的解释.

I am new to assembly.I am reading computers system programmer's perspective. I don't understand how I choose suffix for mov instruction. I know each register and bit count. Suffix usage is determined by bit count (32 bit l, 16 bit w, 8 bit b). Few example is not valid for prior sentence. For example %esp is 32-bit register but for 4. step suffix b is used instead of l. Please give an explanation for using suffix.

问题:

答案:l-w-b-b-l-w-l

资料来源:Bryant, O'Hallaron 的《计算机系统:程序员的视角》(CSAPP)

Source: Computer Systems: A Programmer's Perspective (CSAPP) by Bryant, O'Hallaron

推荐答案

在 movb $-17,(%esp) 中的目标不是寄存器 %esp 而是地址在 %esp 中的内存位置.由于 movb 中的 b，单个字节将存储在该内存位置.存储在那里的值将为 -17(相当于无符号字节 0xef).

In movb $-17,(%esp) the destination is not the register %esp but the memory location whose address is in %esp. Because of the b in movb, a single byte will be stored at that memory location. The value stored there will be -17 (which is equivalent to the unsigned byte 0xef).

movw $-17,(%esp) 和 movl $-17,(%esp) 也是合法的指令，它们会做不同的事情，存储内存位置 %esp 到 %esp+1 或 %esp+3 处的 2 或 4 字节值 0xffef 或 0xffffffef.

movw $-17,(%esp) and movl $-17,(%esp) would also be legal instructions and they'd do different things, storing the 2 or 4 byte values 0xffef or 0xffffffef at memory locations %esp through %esp+1 or %esp+3.

该指令需要 b 或 w 或 l 来消除歧义，这与其他示例不同，因为 $-17 和 (%esp) 是固定大小的实体.如果你尝试 mov $-17,(%esp) 汇编器会报错.

This instruction needs the b or w or l to disambiguate the meaning, unlike your other examples, because neither $-17 nor (%esp) is a fixed-size entity. If you try mov $-17,(%esp) the assembler will complain.

更新:我刚刚注意到问题 #5，push $0xFF，这似乎也可能不明确(pushl $0xFF 和 pushw $0xFF 都是合法的)，但是 push 有一个特殊的规则，只要有歧义就假定 l .16 位推送非常少见(sysv ABI 将堆栈上的所有内容以 4 字节的倍数对齐，因此您始终为函数参数推送 32 位，即使它是 short 或 char)

UPDATE: I just noticed question #5, push $0xFF which also seems like it could be ambiguous (pushl $0xFF and pushw $0xFF are both legal), but there is a special rule for push that assumes l whenever there is an ambiguity. 16-bit pushes are very rare (the sysv ABI keeps everything aligned on the stack in multiples of 4 bytes so you always push 32 bits for a function argument, even if it's a short or char)

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