从数据库1加载字节，但printf整数显示大量 [英] Loading a byte from a DB 1, but printf integer show a large number

问题描述

我正在尝试调用printf来打印一个整数，但不能打印正确的值:

I'm trying to call printf to print an integer, put it doesn't print the right value:

section .data

         an:    db 1
         format: db "num: %d" , 10, 0

section .text
         global main
         extern printf

main:
         push ebp
         mov ebp,esp

         mov eax, [an]
         push eax
         push dword format
         call printf

         add esp, 8
         mov esp,ebp
         pop ebp

         mov eax, 0
         ret

此代码显示"num:1836412417"

this code prints "num: 1836412417"

当我尝试打印一个字符时就可以使用它！

put when I try to print a char it works!

section .data

         an:    db 'a'
         format: db "num: %c" , 10, 0

section .text
         global main
         extern printf

main:
         push ebp
         mov ebp,esp

         mov eax, [an]
         push eax
         push dword format
         call printf

         add esp, 8
         mov esp,ebp
         pop ebp

         mov eax, 0
         ret

现在它显示"num:a"

now it prints "num: a"

那么第一个代码怎么了?！

so what's wrong with the first code ?!!

推荐答案

db 声明8位(一个字节)的值，而％d 打印32位(x86上的四个字节)的值.

db declares 8-bit (one byte) values, while %d prints 32-bit (four byte) values on x86.

实际上，当将32位寄存器 eax 与 mov eax一起加载[an] 时，您正在加载字母"num" 到寄存器的高字节.以后使用％d 将它们打印为数字，或者使用％c 忽略它们.

In effect, when loading 32-bit register eax with mov eax, [an] you are loading bits of letters "num" to high bytes of the register. They are later printed as number, when using %d or ignored when using %c.

要声明32位值，应使用 dd ，而不是 db .

To declare 32 bit values you should use dd, instead of db.

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