Re: [Sdcc-user] PDK calling convention

[Basil H. <ba...@st...>]

On 14/01/2024 14:40, Philipp Klaus Krause wrote:
> The adjustStack helper function in code generation uses add. It works 
> for both negative and nonnegative stack pointer adjutsments, no point 
> writing a version that uses sub.

Fair enough. Was just curious. :)

> Well, padding of unspecified value. Practially, it will often flag 
> bits, since push af tends an efficient efficient way to push a byte as 
> 16-bit value with unspecified upper byte. But this is not guaranteed, 
> and the callee shouldn't rely on it.

Oh, of course. I can't even imagine why anyone would want to look at 
flags previously saved on the stack. So yes, effectively padding, albeit 
of typically non-arbitrary value.

Hmm... what you said made me wonder if, given the flag register is 
writeable, whether you could actually use "push af" to store an 
arbitrary 16-bit value on the stack. But no, the top 4 bits of flag 
aren't writeable, and the docs say they always read as '1', so I presume 
any value in that register will be 0b1111xxxx when saved to the stack. 
An interesting thought experiment nonetheless.

Regards,
Basil Hussain