Re: Question about AMD64 ABI

On Sun, Nov 13, 2022 at 11:53 AM Paul Procacci <pprocacci@gmail.com> wrote:

>
>
> 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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