x86 assembly gcc calling conventions esp register

Question

Documentation says that when assembly method is called first argument should be at 4(%esp). If so is the second argument at 8(%esp)?

I really don't get the gnus at&t assembly syntax so what is this following code equal to in intel syntax?

4(%esp)

Is it [esp + 4] or [esp * 4] for example?

Possible duplicate of http://stackoverflow.com/questions/4003894/what-is-the-0x10-in-the-leal-0x10ebx-eax-x86-assembly-instruction, http://stackoverflow.com/questions/18650093/what-does-a-comma-in-a-parenthesis-mean-in-the-att-syntax-for-x86-assembly, http://stackoverflow.com/questions/26289472/cmp-in-x86-with-parentheses-and-address — Cody Gray - on strike, Dec 30 '16 at 06:57

score 6 · Accepted Answer · answered Jul 18 '14 at 16:15

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4(%esp) is equivalent to [esp + 4], so assuming your first argument is 4 bytes, your second argument should be at 8(%esp)

answered Jul 18 '14 at 16:15

Simon

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score 1 · Answer 2 · answered Jul 18 '14 at 16:16

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depends on type of first argument - different data types require different amounts of space. If you pass an argument which takes more than 4 bytes, there is no way it can be squeezed into a space with the size that the next argument begins 4 bytes beyond first argument.

answered Jul 18 '14 at 16:16

Deleted User

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My assembly method takes two arguments and both are of type `uint8_t` so should I use `[esp + 4]` to retrieve the first argument and `[esp + 12]` for the second? – The amateur programmer Jul 18 '14 at 16:35
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@Theamateurprogrammer no, uint_8 is 8 bits, it may be aligned to 4 bytes, so it's still 8(%esp) – Simon Jul 18 '14 at 16:43
@Simon So I should use `[esp + 4 + nthArgument*argumentLengthInBits]`? – The amateur programmer Jul 18 '14 at 16:59
stack pointer aligned to 4 bytes means, a multiple of 4 bytes is used as displacement. esp+4, esp+8, esp+12... – Deleted User Jul 19 '14 at 07:08
@Bushmills Roger that :) – The amateur programmer Jul 20 '14 at 08:33

x86 assembly gcc calling conventions esp register

2 Answers2