# CSAPP

## chapter 2

#### 整型数值表示

将负数$-x$表示为二进制的方式为: 首先用二进制表示出$+x$, 按位取反后加1即可.

反过来, 依据二进制计算其表示的数字的方法为: 首先依据最高位判断符号, 若最高位为$0$, 则直接利用二进制转十进制确定; 若最高位为$1$, 则确定符号为负, 其绝对值由减一后按位取反, 再用二进制转十进制确定.

**练习**

假定`INT`类型为1字节有符号整数, 则数值表示范围为$\[-2^7,2^7-1]=\[-128,127]$.

+10 = 00001010 => 11110101 => -10 = 11110110 => 256 + (-10) = 246 = 11110110

11110110 => - \~(11110101)=>- 00001010 => -10

#### 浮点型数值表示

## chapter 3

```c
//mstore.c
long mult2(long, long);
void multstore(long x, long y, long *dest) {
long t = mult2(x, y);
*dest = t;
}
```

```
$ gcc -Og -S mstore.c
$ gcc -c -Og -o mstore.o mstore.c
```

显示.o文件里的部分机器码:

```
$ gdb mstore.o
(gdb) x/14xb multstore # 显示机器码0x0 <multstore>:    0x53    0x48    0x89    0xd3    0xe8    0x00    0x00    0x00
0x8 <multstore+8>:    0x00    0x48    0x89    0x03    0x5b    0xc3
```

显示.s文件里的内容:

```
//mstore.s
    .file    "mstore.c"
    .text
    .globl    multstore
    .type    multstore, @function
multstore:
.LFB0:
    .cfi_startproc
    pushq    %rbx
    .cfi_def_cfa_offset 16
    .cfi_offset 3, -16
    movq    %rdx, %rbx
    call    mult2
    movq    %rax, (%rbx)
    popq    %rbx
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE0:
    .size    multstore, .-multstore
    .ident    "GCC: (GNU) 5.2.0"
    .section    .note.GNU-stack,"",@progbits
```

反汇编mstore.o:

```bash
$ objdump -d mstore.o

mstore.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <multstore>:
   0:    53                       push   %rbx
   1:    48 89 d3                 mov    %rdx,%rbx
   4:    e8 00 00 00 00           callq  9 <multstore+0x9>
   9:    48 89 03                 mov    %rax,(%rbx)
   c:    5b                       pop    %rbx
   d:    c3                       retq
```

##


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://buxianchen.gitbook.io/notes/learn/csapp.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.