汇编（x86）：＆lt; label＆gt; db＆＃x27; string＆＃x27;，除非有SA跳转指令，否则不会执行0

发布于 2025-01-27 05:46:45 字数 2418 浏览 2 评论 0原文

我一直在墙上敲打头，以了解为什么以下组件没有正确地倾倒“ Hello_world”的内容。

; Explicitly set 16-bit
[ BITS 16 ]
[ ORG 0x7C00 ]

; Create label for hello world string terminated by null.
HELLO_WORLD db 'hello world', 0

start:
    ; Move address of HELLO_WORLD into si
    mov SI, HELLO_WORLD
    call print_string

    ; Continue until the end of time
    jmp $

print_string:
    loop:
        ; Retrieve value stored in address at si
        mov al, [SI]
        mov ah, 0x0E
        cmp al, 0
        ; Finish execution after hitting null terminator
        je  return
        INT 0x10
        ; Increment contents of si (address)
        inc SI
        jmp loop

    return:
        ret

; boot loader length *must* be 512 bytes.
times 510-($-$$) db 0
dw 0xAA55

最后，我发现，如果我们不执行（不是代码）标签，则它可以正常运行。

jmp start
HELLO_WORLD db 'hello world',0

我发现最令人困惑的部分，看着十六进制转储，Hello_world仍在二进制中（在开始时 - 似乎没有区别）。

cat nojmp_boot.out

00000000  68 65 6c 6c 6f 20 77 6f  72 6c 64 00 be 00 7c e8  |hello world...|.|
00000010  02 00 eb fe 8a 04 b4 0e  3c 00 74 05 cd 10 46 eb  |........<.t...F.|
00000020  f3 c3 eb e8 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

cat jmpboot.out

00000000  eb 22 68 65 6c 6c 6f 20  77 6f 72 6c 64 00 be 02  |."hello world...|
00000010  7c e8 02 00 eb fe 8a 04  b4 0e 3c 00 74 05 cd 10  ||.........<.t...|
00000020  46 eb f3 c3 eb e8 00 00  00 00 00 00 00 00 00 00  |F...............|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

检查前两个字节，我们可以看到'e8 22'是一个近亲，可以解决22（“ nofollow”>“ nofollow”> ion_overview.pdf ）。

我的问题是：

为什么我们不能将“ Hello_world”作为程序执行的一部分，就我而言，代码和数据之间没有区别？

我正在使用以下汇编：

nasm -f bin -o boot.bin boot.asm && if [ $(stat -c "%s" boot.bin) -ne 512 ]; then x; fi && qemu-system-x86_64 boot.bin

原文

I've been banging my head against the wall in an attempt to understand why the following assembly is not correctly dumping the contents of 'HELLO_WORLD'.

; Explicitly set 16-bit
[ BITS 16 ]
[ ORG 0x7C00 ]

; Create label for hello world string terminated by null.
HELLO_WORLD db 'hello world', 0

start:
    ; Move address of HELLO_WORLD into si
    mov SI, HELLO_WORLD
    call print_string

    ; Continue until the end of time
    jmp $

print_string:
    loop:
        ; Retrieve value stored in address at si
        mov al, [SI]
        mov ah, 0x0E
        cmp al, 0
        ; Finish execution after hitting null terminator
        je  return
        INT 0x10
        ; Increment contents of si (address)
        inc SI
        jmp loop

    return:
        ret

; boot loader length *must* be 512 bytes.
times 510-($-$) db 0
dw 0xAA55

In the end, I discovered that if we do not execute (make it not code) the label, then it functions correctly.

jmp start
HELLO_WORLD db 'hello world',0

The part I find the most confusing, looking at the hex dump, HELLO_WORLD is still in the binary (at the beginning - and there appears to be no distinction of its type).

cat nojmp_boot.out

00000000  68 65 6c 6c 6f 20 77 6f  72 6c 64 00 be 00 7c e8  |hello world...|.|
00000010  02 00 eb fe 8a 04 b4 0e  3c 00 74 05 cd 10 46 eb  |........<.t...F.|
00000020  f3 c3 eb e8 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

cat jmpboot.out

00000000  eb 22 68 65 6c 6c 6f 20  77 6f 72 6c 64 00 be 02  |."hello world...|
00000010  7c e8 02 00 eb fe 8a 04  b4 0e 3c 00 74 05 cd 10  ||.........<.t...|
00000020  46 eb f3 c3 eb e8 00 00  00 00 00 00 00 00 00 00  |F...............|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

Inspecting the first two bytes, we can see 'e8 22' is a shortjump to address 22 (http://net.cs.uni-bonn.de/fileadmin/user_upload/plohmann/x86_opcode_structure_and_instruction_overview.pdf).

My question is:

Why can we not have 'HELLO_WORLD' as a part of the execution of the program, as far I was concerned, there was no distinction between code and data?

I'm using the following for compilation:

nasm -f bin -o boot.bin boot.asm && if [ $(stat -c "%s" boot.bin) -ne 512 ]; then x; fi && qemu-system-x86_64 boot.bin

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亣腦蒛氧 2025-02-03 05:46:45

执行从顶部开始。如果您省略了JMP启动，则字符 h 将被CPU解释，就好像它是指令一样。当然，您认为这是不正确的吗？

就我而言，代码和数据之间没有区别？

当我们考虑二进制中的放置时，代码和数据之间没有区别。但是代码和数据仍然保留2个完全不同的项目。代码是唯一可以由CPU执行的代码。

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最美的太阳 2025-02-03 05:46:45

由于您正在创建引导扇区，因此执行始于生成文件的第一个字节。它不会从开始标签或其他任何地方开始。由于字符串“ Hello World”是在文件的开头，因此这些字节首先被执行。这些字节由CPU解释为说明，而不是字符，并且它们被执行为被解码为解码的任何说明。

以下是执行的说明：

7c00:   68 65 6c                push   0x6c65
7c03:   6c                      ins    BYTE PTR es:[di],dx
7c04:   6f                      outs   dx,WORD PTR ds:[si]
7c05:   20 77 6f                and    BYTE PTR [bx+0x6f],dh
7c08:   72 6c                   jb     0x7c76
7c0a:   64 00 be 00 7c          add    BYTE PTR fs:[bp+0x7c00],bh
7c0f:   e8 02 00                call   0x7c14
7c12:   eb fe                   jmp    0x7c12
7c14:   8a 04                   mov    al,BYTE PTR [si]
...

Since you're creating a boot sector the execution begins at the first byte of the generated file. It won't begin at the start label or anywhere else. Since the string "hello world" is at the start of the file these bytes are what get executed first. These bytes are interpreted by the CPU as instructions, not characters, and they get executed as whatever instructions they get decoded as.

Here are the instructions that get executed:

7c00:   68 65 6c                push   0x6c65
7c03:   6c                      ins    BYTE PTR es:[di],dx
7c04:   6f                      outs   dx,WORD PTR ds:[si]
7c05:   20 77 6f                and    BYTE PTR [bx+0x6f],dh
7c08:   72 6c                   jb     0x7c76
7c0a:   64 00 be 00 7c          add    BYTE PTR fs:[bp+0x7c00],bh
7c0f:   e8 02 00                call   0x7c14
7c12:   eb fe                   jmp    0x7c12
7c14:   8a 04                   mov    al,BYTE PTR [si]
...

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