[var] 和 var 之间的汇编差异 [英] Assembly difference between [var], and var

问题描述

我正在学习汇编程序，但实际上我对 [variable] 和 variable 之间的区别一无所知.正如教程所说，两者都是指针，那么这有什么意义呢?为什么我必须在 [] 之前使用 type Identifier ?我的汇编程序:nasm x86_64 在 Linux 上运行-->Ubuntu

I'm learning Assembler and getting to the point where I actually have no clue about the difference between [variable] and variable. As the tutorials say, both are pointers, so what is the point of this? And why do I have to use a type Identifier before []? my assembler: nasm x86_64 running on Linux--> Ubuntu

推荐答案

在 x86 Intel 语法中 [expression] 表示地址 expression 处的内存内容.
(除了在 MASM 中，当 expression 是没有寄存器的数字文字或 equ 常量时，那么它仍然是立即的)

In x86 Intel syntax [expression] means content of memory at address expression.
(Except in MASM when expression is a numeric literal or equ constant with no registers, then it's still an immediate)

expression 不带括号取决于您使用的汇编程序.

expression without brackets depends on Assembler you are using.

NASM 风格(NASM、YASM):

NASM-style (NASM, YASM):

mov eax,variable      ; moves address of variable into eax
lea eax,[variable]    ; equivalent to the previous one (LEA is exception)
mov eax,[variable]    ; loads content of variable into eax

MASM 风格(还有 TASM 甚至 GCC/GAS .intel_syntax noprefix):

MASM-style (also TASM and even GCC/GAS .intel_syntax noprefix):

mov eax,variable      ; load content of variable (for lazy programmers)
mov eax,OFFSET variable   ; address of variable
lea eax,[variable]    ; address of variable
mov eax,[variable]    ; content of variable

GAS(AT&T 语法):这不是 Intel 语法，请参阅AT&T 标签维基.GAS 还使用不同的指令(例如 .byte 而不是 db)，即使在 .intel_syntax 模式下也是如此.

GAS (AT&T syntax): It's not Intel syntax, see the AT&T tag wiki. GAS also uses different directives (like .byte instead of db), even in .intel_syntax mode.

在所有情况下，variable 都是符号的别名，用于标记内存中出现标签的特定位置.所以:

In all cases the variable is alias for symbol marking particular place in memory, where the label appeared. So:

variable1  db  41
variable2  dw  41
label1:

在符号表中产生三个符号，variable1、variable2和label1.

produces three symbols into symbol table, variable1, variable2 and label1.

当你在代码中使用它们中的任何一个时，比如 mov eax,，它没有任何信息是由 db 或 定义的dw 或作为标签，所以当你做 mov [variable1],ebx(覆盖定义的第一个字节之外的 3 个字节)时它不会给你任何警告.

When you use any of them in the code, like mov eax,<symbol>, it has no information whether it was defined by db or dw or as label, so it will not give you any warning when you do mov [variable1],ebx (overwriting 3 bytes beyond the defined first byte).

它只是内存中的一个地址.

It's simply just an address in memory.

(MASM 除外，其中数据部分中标签后的 db 或 dd 确实将大小与其变量名称"相关联.)

(Except in MASM, where the db or dd after a label in a data section does associate a size with it that "variable name".)

当不能从指令操作数本身推导出类型时，类型标识符仅在大多数汇编器中是必需的.

Type identifier is only required in most of the assemblers when the type can't be deduced from the instruction operands itself.

mov [ebx],eax ; obviously 32 bits are stored, because eax is 32b wide
mov [ebx],1   ; ERROR: how "wide" is that immediate value 1?
mov [ebx],WORD 1 ; NASM syntax (16 bit value, storing two bytes)
mov WORD [ebx],1 ; NASM syntax (16 bit value, storing two bytes)
mov WORD PTR [ebx],1 ; MASM/TASM syntax

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