ex1 了解汇编

命令gcc -S -m32 lab0_ex1.c
-S把文件编译成为汇编代码
-m32编译成32位程序，在64位机器需要指定
lab0_ex1.c下的内容

int count=1;
int value=1;
int buf[10];
void main()            
{                
   asm(
        "cld \n\t"
        "rep \n\t"                         
        "stosl"
    :
    : "c" (count), "a" (value) , "D" (buf[0])
    :
      );
}
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cldclear direction 清除方向标志位（DF），DF=0表示指令执行过程中源变址寄存器（ESI）/目的变址寄存器（EDI）往增达方向移动
rep 当计数寄存器（ECX）不为零,ECX=ECX-1并重复执行：：
stosl stos指令是字符串处理指令的一种，即Store String Instruction，将EAX中的值保存到ES:EDI指向的地址中，根据方向标志位DF决定EDI+4/-4

这几条语句的功能是向buf中写上count个value值

asm内联语句部分符号含义
c-ECX
a-EAX
D-EDI
结果(lab0_ex1.s)

    .file    "lab0_ex1.c"
    .globl    count
    .data
    .align 4
    .type    count, @object
    .size    count, 4
count:
    .long    1
    .globl    value
    .align 4
    .type    value, @object
    .size    value, 4
value:
    .long    1
    .comm    buf,40,32
    .text
    .globl    main
    .type    main, @function
main:
.LFB0:
    .cfi_startproc
    pushl    %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    pushl    %edi
    pushl    %ebx
    .cfi_offset 7, -12
    .cfi_offset 3, -16
    movl    count, %edx
    movl    value, %eax
    movl    buf, %ebx
    movl    %edx, %ecx
    movl    %ebx, %edi
#APP
# 6 "lab0_ex1.c" 1
    cld 
    rep 
    stosl
# 0 "" 2
#NO_APP
    popl    %ebx
    .cfi_restore 3
    popl    %edi
    .cfi_restore 7
    popl    %ebp
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc
.LFE0:
    .size    main, .-main
    .ident    "GCC: (Ubuntu 4.8.2-19ubuntu1) 4.8.2"
    .section    .note.GNU-stack,"",@progbits
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CFI 即 Call Frame Information，是 DWARF 2.0 定义的函数栈信息，DWARF 即 Debugging With Attributed Record Formats ，是一种调试信息格式。 .cfi_ 开头的汇编指示符用来告诉汇编器生成相应的 DWARF 调试信息，主要是和函数有关。.cfi_startproc 定义函数开始，.cfi_endproc 定义函数结束。
原文链接：https://blog.csdn.net/zoomdy/article/details/80700750
cfi指令解析：https://sourceware.org/binutils/docs/as/CFI-directives.html、 http://www.dwarfstd.or/doc/DWARF5.pdf

ex2 用gdb调试

命令：gcc -g -m32 lab0_ex2.c -g生成供调试用的可执行文件，可以在gdb中运行
lab0_ex2.c 打印“hello world”字符串

#include <stdio.h>
int
main(void)
{
    printf("Hello, world!\n");
    return 0;
}
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进入加载可执行文件a.out，启动gdb调试：gdb a.out

ex3 掌握指针和类型转换相关的Ｃ编程

命令：gcc -g -m32 lab0_ex3.c 2>&1|tee make.log 2>&1 将标准错误输出重定向到标准输出 tee 从标准输入设备读取数据，将其内容输出到标准输出设备，同时保存成文件（make.log） lab0_ex3.c内容：

int
main(void)
{
    unsigned before;
    unsigned intr;
    unsigned after;
    struct gatedesc gintr;
    
    intr=8;
    before=after=0;

    gintr=*((struct gatedesc *)&intr);
    SETGATE(gintr, 0,1,2,3);
    intr=*(unsigned *)&(gintr);
    printf("intr is 0x%x\n",intr);
    printf("gintr is 0x%llx\n",gintr);
    
    return 0;
}
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报错——类型有问题:

lab0_ex3.c: In function \u2018main\u2019:
lab0_ex3.c:48:5: warning: format \u2018%llx\u2019 expects argument of type \u2018long long unsigned int\u2019, but argument 2 has type \u2018struct gatedesc\u2019 [-Wformat=]
     printf("gintr is 0x%llx\n",gintr);
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修改一： printf("gintr is 0x%llx\n",(long long unsigned)gintr); 继续报错：

lab0_ex3.c: In function \u2018main\u2019:
lab0_ex3.c:48:5: error: aggregate value used where an integer was expected
     printf("gintr is 0x%llx\n",(long long unsigned)gintr);
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原因： C语言的类型强制转换只有在数量类型之间才能进行转换，结构类型（包括 union，struct）不是数量类型（Scalar Type）,无法进行强制转换； 数量类型包括算术类型和指针类型，算术类型由包括整数类型和浮点类型。（https://blog.csdn.net/swj9099/article/details/81359690） 修改二： printf("gintr is 0x%llx\n",*(long long unsigned*)&gintr); 得到结果（intr为32为整形，高位0省去，gintr为long long int 为64位长整型）

intr is 0x10002
gintr is ee0000010002
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打印的gintr为gatedesc结构体（中断门描述符）经过SETGATE(gintr, 0,1,2：,3);赋值后的结果(intr为低32位的结果)，结构体gatedesc和SETGATE宏定义为：

#define STS_IG32        0xE            // 32-bit Interrupt Gate
#define STS_TG32        0xF            // 32-bit Trap Gate
typedef unsigned uint32_t;
#define SETGATE(gate, istrap, sel, off, dpl) {            \
    (gate).gd_off_15_0 = (uint32_t)(off) & 0xffff;        \
    (gate).gd_ss = (sel);                                \
    (gate).gd_args = 0;                                    \
    (gate).gd_rsv1 = 0;                                    \
    (gate).gd_type = (istrap) ? STS_TG32 : STS_IG32;    \
    (gate).gd_s = 0;                                    \
    (gate).gd_dpl = (dpl);                                \
    (gate).gd_p = 1;                                    \
    (gate).gd_off_31_16 = (uint32_t)(off) >> 16;        \
}

 /* Gate descriptors for interrupts and traps */
 struct gatedesc {
    unsigned gd_off_15_0 : 16;        // low 16 bits of offset in segment
    unsigned gd_ss : 16;            // segment selector
    unsigned gd_args : 5;            // # args, 0 for interrupt/trap gates
    unsigned gd_rsv1 : 3;            // reserved(should be zero I guess)
    unsigned gd_type : 4;            // type(STS_{TG,IG32,TG32})
    unsigned gd_s : 1;                // must be 0 (system)
    unsigned gd_dpl : 2;            // descriptor(meaning new) privilege level
    unsigned gd_p : 1;                // Present
    unsigned gd_off_31_16 : 16;        // high bits of offset in segment
 };
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```