Re: Ultimax mode VIC II access to external RAM?

Hello!

Jim Brain wrote:

> Ultimax mode is clearly one of the most confusing (and thus interesting)
> modes on the C64. 
> I see that one can populate external RAM on the cart bus, and access it
> in Ultimax mode.  But, the documents don't seem to indicate if the
> VIC-II an "see" that RAM in the mode. 

I am not sure what kind of RAM you mean here.

Ultimax has 2 kB of RAM, and the cartridge port has a decode line for
additional 2 kB (in the C64 this line os repurposed for $DF00 I/O). I
have not personally tested whether VIC can access this additional RAM,
but I don't see any reason why it wouldn't (it could be easily tested
using the MAX Basic cartridge, which contains this extra RAM). The C64,
however, always provides 4 kB of RAM in Ultimax mode, so this extra RAM
on cartridge is never used even if it is attached.

Or do you perhaps mean attaching RAM in place of ROM chips? In that
case, VIC does not normally see these ROM chips so it will not see RAM
there as well (except, of course, the area hardwired to $3000).

Or do you mean attaching additional RAM in the unused address space, for
example $1000-$2FFF (for which you must do your own address decoding)?
Then I don't know exactly whot would happen, but my gut tells me that in
the real Ultimax VIC would see this RAM, but in the C64 it might or
might not (my bet is on "not").

Not very informative, but without building the actual hardware it's all
guesswork at this point.

Regards,
Michau.





> 
> http://codebase64.org/doku.php?id=base:memmanage
> 
> However, this is not true for the Ultimax video game
> configuration. In that mode, the internal RAM ignores all
> memory accesses outside the area $0000-$0FFF, unless they
> are performed by the VIC, and you can write to external
> memory at $1000-$CFFF and $E000-$FFFF, if any, without
> changing the contents of the internal RAM.
> 
> Marko noted long ago that he was aware of a page the VIC could read
> from, and there is this note on the list archives from 1999:
> 
> http://cbm-hackers.2304266.n4.nabble.com/lt-Q-gt-C64-ULTIMAX-mode-td4059331.html
> 
> Does anyone have more information?
> 
> Also, on this portion of the archive:
> 
> http://cbm-hackers.2304266.n4.nabble.com/Ultimax-questions-td4615195.html
> 
> skoe notes that he was leery of just decoding the address lines because
> of the state of $01.  How would the state of $01 be of interest or
> concern in Ultimax mode?
> 
> Jim
> 
> -- 
> 
> Jim Brain
> brain@jbrain.com 
> www.jbrain.com
>