Re: SuperCPU for my C64 and C128

Thank you  Mia,

I didn't know much about the the Max or Ultimax model. The information that
you shared and ideas, is great in order to establish the possibilties of
dealing with the RAM resources. I Will check the _game technique.

Thanks,
Luis Vela
dsp8bit


El jue., 19 de jul. de 2018 6:37 PM, Mia Magnusson <mia@plea.se> escribió:

> Den Wed, 18 Jul 2018 13:57:48 -0500 skrev Luis Rene Vela Garcia
> <lrvg1010@gmail.com>:
> > Hi Guys,
>
> Welcome :)
>
> I'm not totally sure exact what information you are looking for.
>
> But in general there are two strategies here. One needs to be used and
> the other can greatly speed up things.
>
> As Ruuds stuff about the CP/M cartridge says, the C64 has a mode where
> an external CPU can access the C64's internal memory.
>
> But you can also do it the other way around, let the C64 access
> external memory. This is called the "Max" or "Ultimax" mode.
>
> The intention of the mode is to provide compatibility with the
> Max/Ultimax gamle machine which was like a C64 but almost only for game
> playing, containing a small amount of static ram and partially the I/O
> a C64 has, but otherwise you had to provide a cartridge with rom and in
> some cases (the Basic programming language cartridge) also ram).
>
> But in practise you can assert _GAME at any time which disables most of
> the C64 internal memory and let you drive the data bus in the cartridge
> port when the C64 does a read operation. This way you can provide
> external memory for the VIC-II video chip. The exception is the first
> part of RAM (iirc 4k) which were actual ram on a Max/Ultimax and thus
> the internal C64 memory is always enabled at those addresses. Also the
> I/O areas might not be possible to disable (but I'm not sure here).
>
> For a 65816 accelerator I think you should start as Ruud suggest by
> running the 65816 at 1MHz and let it use the C64's internal memory for
> the first 64k of address space.
>
> Then I actually suggest using a PLL driven oscillator and sync it to
> the C64 clock frequency as it makes timing easier.
>
> The next step would be to add ram which at first is enabled for the
> addresses above 64k.
>
> Here comes a nice trick: The next step would be to put the C64 in
> Max/Ultimax mode and let the C64 VIC-II chip read from the faster
> memory in the 65816. From the point where VIC-II supplies a valid
> address to the point it samples the bus you are free to do a faster
> read cycle of your memory and latch the read data until the VIC-II
> actually samples the data bus. That way a VIC-II read cycle will only
> take as short time as the memory can provide data at, and not a full
> half of a 1MHz cycle.
>
> Unfortunitely this won't work for the first 4k. You can speed up things
> a lot by writing both to the internal C64 memory and the faster
> expansion memory, but only read from the faster memory. There is
> nothing but the CPU that can alter memory contents so that won't cause
> any incompatibly.
>
> The I/O address space must of course be directed directly to the
> existing C64 hardware.
>
> A tricky part is the 6510 buildt-in I/O port at address 0/1. The only
> way to actually read/write to that port is to run code in the 6510
> itself. However that port controls memory mapping and some pins of the
> cassette interface. If we assume the 65816 won't use the cassette
> interface (but switch to 6510 mode as the cassette port anyway needs
> compatible timing) the memory mappings can be simulated by adding a
> port at address 0/1 on the 65816 and map in/out write protected parts
> of memory that contains copies of the C64 ROM's, and also map in/out
> access to the original C64 hardware for the I/O space. That way almost
> all C64 software will run fine, and you can also soft load basic/kernal
> rom images.
>
> I'm not sure how JiffyDOS works on a SuperCPU, but in general JiffyDOS
> is a replacement kernal so you'd just need to use another image for
> Kernal. If you want to, you could make your hardware able to show it's
> ram to the 6510 CPU (using the _GAME technique) making it possible to
> let the 6510 use a JiffyDOS image instead of the standard kernal.
>
> Btw even thogh JiffyDOS is a great improvement, as the 65816 mode won't
> be compatible with existing timing critical software (like games and
> demos) anyways, I suggest adding a SD card interface to your hardware,
> and space for some kind of ROM making it possible to load/save to the
> SD card directly. That would win over any JiffyDOS solution speedwise,
> while those wanting a 100% compatible drive can use either an existing
> 1541 or a 1541 Ultimate, even together with your hardware.
>
>
>
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>