Assembly (x86): db 'string',0 does not get executed unless there's a jump instruction

Question

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\'.

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Answer 1

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]
...

Answer 2

Execution starts at the top. If you omit the jmp start then the character h will get interpreted by the CPU as if it were an instruction. Surely you see that such can not be correct?

as far I was concerned, there was no distinction between code and data?

There's no distinction between code and data when we consider their placement in the binary. But code and data still remain 2 completly different items. Code being the only one that can get executed by the CPU.