[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，直到我对[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? 我的汇编器:nasm x86_64 running on Linux--> 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中，Intel语法[expression]表示地址expression处的内存内容.
(在MASM中，当expression是数字文字或没有寄存器的equ常量时，那仍然是即时的)

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器.

expression without brackets depends on Assembler you are using.

NASM样式(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样式(也包括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，请参见 AT& T标签Wiki .即使在.intel_syntax模式下，GAS也会使用不同的指令(例如.byte而不是db).

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variable1，variable2和label1中产生三个符号.

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

当您在代码中使用它们中的任何一个时，例如mov eax,<symbol>，它不知道它是由db还是dw定义还是作为标签，因此当您mov [variable1],ebx(在定义的第一个字节后覆盖3个字节).

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).

这只是内存中的一个地址.

It's simply just an address in memory.

(在MASM中，数据部分中标签后面的db或dd确实将大小与变量名"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".)

在大多数汇编器中仅需类型标识符.

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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