What is the meaning of parentheses in opcodes in a NASM generated listing file?
assembly, instructions, nasm, opcode
Solution
They're showing where relocations will be applied at link time.
`[nnnnnnnn]` shows an absolute relocation (when the linker performs the relocation, the base address of some section will be added to the offset). e.g. once the binary is fully linked, those bytes in the `push str2` instruction will be changed to contain the base address of `.data` + `0x09`.
`(nnnnnnnn)` shows a PC-relative relocation (used for calls and branches, where the final value must be relative to the address of the next instruction). e.g. the bytes in the `call func2` instruction will be updated with the difference between the final address of `func2`, and the address of the following instruction (`call func3`).
There are no brackets for the `call func3` instruction itself because no relocation is needed in that case - the `call` instruction needs a relative address, but `func3` is within the same section, so the relative address is already known (the following instruction is at `.text + 0x21`, and `func3` is at `.text + 0x2c`, so the relative address is `0xb` regardless of the final address of `.text`).
Problem
When looking at a listing file that was generated by NASM, I see that there are three kinds of opcodes: - Without parentheses. - With round parentheses. - With square parentheses. What they mean? when each of them is used? This is an example of a listing file that demonstrate all of the above: ``` 1 section .text 2 extern printf 3 extern fgets 4 00000000 313233 str3: db "123" 5 main: 6 00000003 68[00000000] push str1 7 00000008 68[09000000] push str2 8 0000000D 68[00000000] push str3 9 00000012 E8(00000000) call func1 10 00000017 E8(04000000) call func2 11 0000001C E80B000000 call func3 12 00000021 E8(00000000) call printf 13 00000026 E8(00000000) call fgets 14 0000002B C3 ret 15 16 0000002C 90 func3: nop 17 0000002D C3 ret 18 19 20 section .text1 21 00000000 90 func1: nop 22 00000001 90 nop 23 00000002 90 nop 24 00000003 C3 ret 25 26 00000004 90 func2: nop 27 00000005 90 nop 28 00000006 90 nop 29 00000007 C3 ret 30 31 32 section .data 33 34 00000000 313233343536373839 str1: db "123456789" 35 00000009 313233343536373839 str2: db "123456789" ```