汇编学习笔记

本文主要记录汇编学习笔记。

RAX,EAX,AX,AH,AL

RAX是64位的寄存器
EAX是32位的寄存器，而AX是EAX的低16位。
AH是AX的高8位，而AL是AX的低8位

操作数前缀

1
2
3

movl   $8,%eax 
movl   $0xffff,%ebx 
int     $0x80

看到在AT%T汇编中诸如”%eax”、”%ebx”之类的寄存器名字前都要加上”%”；”$8”、”$0xffff”这样的立即数之前都要加上”$”。

源/目的操作数顺序

在Intel语法中，第一个操作数是目的操作数，第二个操作数源操作数。而在AT&T中，第一个数是源操作数，第二个数是目的操作数。

// INTEL语法
MOV EAX,8 //EAX是目的操作数， 8是源操作数
// AT&T语法
movl   $8,%eax //8是源操作数 EAX是目的操作数

标识长度的操作码后缀

在AT&T的操作码后面有时还会有一个后缀，其含义就是指出操作码的大小。“l”表示长整数（32位），“w”表示字（16位），“b”表示字节（8位）。

movb    %bl,%al
movw   %bx,%ax
movl     %ebx,%eax
movl     (%ebx),%eax

Labels

A label can be placed at the beginning of a statement. During assembly, the label is assigned the current value of the active location counter and serves as an instruction operand. There are two types of lables: symbolic and numeric.

Symbolic Labels

A symbolic label consists of an identifier (or symbol) followed by a colon (:) (ASCII 0x3A). Symbolic labels must be defined only once. Symbolic labels have global scope and appear in the object file’s symbol table.

Symbolic labels with identifiers beginning with a period (.) (ASCII 0x2E) are considered to have local scope and are not included in the object file’s symbol table.

Numeric Labels

A numeric label consists of a unsigned decimal int32 value followed by a colon (:). Numeric labels are used only for local reference and are not included in the object file’s symbol table. Numeric labels have limited scope and can be redefined repeatedly.

When a numeric label is used as a reference (as an instruction operand, for example), the suffixes b (“backward”) or f (“forward”) should be added to the numeric label. For numeric label N, the reference Nb refers to the nearest label N defined before the reference, and the reference Nf refers to the nearest label N defined after the reference. The following example illustrates the use of numeric labels:

1:          / define numeric label "1"
one:        / define symbolic label "one"

/ ... assembler code ...

jmp   1f    / jump to first numeric label "1" defined
            / after this instruction
            / (this reference is equivalent to label "two")

jmp   1b    / jump to last numeric label "1" defined
            / before this instruction
            / (this reference is equivalent to label "one")

1:          / redefine label "1"
two:        / define symbolic label "two"

jmp   1b    / jump to last numeric label "1" defined
            / before this instruction
            / (this reference is equivalent to label "two")

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