Re: Question about AMD64 ABI

On Sat, Nov 12, 2022 at 10:31 PM Daniel Cervus <DanieltheDeer@outlook.com>
wrote:

> Hi everyone,
>
> I’m trying to do assembly programming on FB in 64-bit mode. I have a
> question, 64-bit mode requires parameters to be passed on 64-bit registers.
> But when a parameter is 32-bit or smaller, do I need to sign-extend (or
> zero-extend) them to 64-bit? The System V ABI specifications only says "The
> size of each argument gets rounded up to eightbytes." It’s somewhat
> ambiguous. How to round up 'float', when they are passed on stack?
>
> Thanks,
> Daniel


(Didn't Reply all)

Hi Daniel,

There are a handful of operations that operate on 32bit registers that
automatically clear the high bits for you.

32-bit operands generate a 32-bit result, zero-extended to a 64-bit result
in the destination general-purpose register.
16 and 8 bit operands don't have this "built-in" so you would indeed need
to ensure the higher bits are cleared.

mov dword edi, 1  is effectively setting rdi to the value of
0x0000000000000001

As for sign extending the values, the answer is `no'.....under most
circumstances.  If you are sticking to widths of 32 and 64 bits then you
are fine.
The moment you mess with 16bits or smaller, then yes, you need to ensure no
garbage lives in your higher bits because the cpu doesn't clear this for
you.

"The size of each argument gets rounded up to eight bytes."

The size of ALL arguments passed to the callee via general purpose
registers is 8 bytes "regardless of what a function def says".  It's HOW
the callee operates upon the register arguments that matters.

As for passing arguments on the stack ...  you shouldn't have to.  Not only
are there the GPR's rdi, rsi, rdx, rcx, r8, r9, r10 at your disposal for
int/scalar types there are also xmm0-xmm7 for your floats.

Thanks,
~Paul

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