Re: SuperCPU for my C64 and C128

On 24/07/2018 23:56, Mia Magnusson wrote:
>
> We are talking about VIC reading from fast ram here, aren't we? :O

Yes. Which on SuperCPU is only ever accessed by the 65816 reading and 
writing and the supercpu only uses the expansion port for updating the 
mirrored ram inside the c64 and for reading I/O.

Implementing ultimax it then has to monitor the expansion port for vic2 
reads and be able to satisfy them from the fast ram. It pretty much 
doubles the complexity.

> Well, it's true that you have a software selectable 1MHz mode, but the
> memory hardware will itself slow down the CPU when the CPU accesses
> 1MHz address space (i.e. the first 4k if not using shadow, and all I/O).

You can't switch to 1mhz for the first 4k, that is where zero page and 
stack are.

Most software isn't going to be using the first two pages for vic, but I 
remember writing basic games which used the tape buffer etc for sprites. 
So it would make sense to use some of the reserved v2 optimisation 
register values to add a mode where $200-$7ff is mirrored & one where 
$200-$1000 is mirrored.

> What SuperCPU software are there that really uses exact timing?

I don't have a SuperCPU, but in the CMD FAQ it talks about writing FLI 
in 20mhz. FLI is going to be timing sensitive.

I don't understand why the first thought on making a SuperCPU clone 
would be that it's not worth doing due to lack of existing software, so 
you might as well make something entirely different.

> But yeah, as VIC-II anyway will use C64's internal ram for the first
> 4k, all logic to handle not using my proposal is already there so it
> would be super easy to add an on/off function.

As I pointed out, you don't want the SuperCPU to use the first 4k.

The logic isn't already there, he hasn't even started implementing it 
AFAICT.

I quite like the idea of using ultimax mode, but it would seem to be 
something you'd do after you'd got the existing functionality working.

Especially as he wants to support C128 mode as well, which adds another 
level of complexity & there is less documentation on how that works.