Question

I’m very new in x86 Assembly Language. I’m reading a book called pcasm and I was wondering if someone can help me to understand this code example better (It’s partial code from the book):

32    mov    ebx, input2
33    mov    ecx, $ + 7
34    jmp    short get_int
35
36    mov    eax, [input1]


64    get_int:
65        call   read_int
66        mov    [ebx], eax
67        jmp    ecx

Now, what I understand is $ gives the current address, but:

1. Why + 7?
2. How can I calculate it?
3. What would happen to the number if I use jmp near get_int (4 bytes) and jmp near word get_int (2 bytes)? Is the second syntax correct or it should be jmp word get_int?

Thanks

Answer 1

1. The example code uses + 7 because presumably there are 7 bytes of machine code generated for source lines 33 and 34 combined.

2. You can calculate the offset needed by looking at the assembler output listing (which is something you might have to turn on in your assembler) and counting the bytes.

3. If you use instructions that assemble to a different number of machine code bytes, then the required offset will be different. You’d have to try it in your environment to see what you need.