Re: Theorizing how to replace a VIC-II by hardware.

Hi,

I am currently working on the same with TED. I have already put together my
proof of concept and works fine. I have implemented all TED input/outputs
but the analog parts (Chroma, Luma) need some external components (not
much). I am using FPGA and currently the biggest challenge is the PCB
planning to fit it in the plus4's metal shield. I could easily do it with
a small BGA chip but that requires special PCB design and might be
expensive.
So what you need is an FPGA core with the analog parts implemented. I am
sure someone is already working on it.

Istvan



On Fri, Jul 24, 2020 at 8:48 PM Claudio Sánchez <tokafondo_at_gmail.com> wrote:

> Well... I'm starting this as a discussion about what would be needed to
> replace the VIC-II with hardware that would emulate it and
> expand/enhance it.
>
> For a start, I'm a reader. I read, ideas comes to my mind and then I
> ask, so I can learn from people that actually know.
>
> So...
>
> AFAIK (and I'm telling this for people to know what is what I do know,
> accepting corrections of course) , the VIC-II do cares about
>
> - Display text and graphics.
> - DRAM refresh.
> - System clock generation.
>
> --For the first, it reads from the CHAR ROM for text, or from RAM when
> instructed to do so, mostly by KERNAL but also by custom routines. For
> graphics, it reads from RAM, and is the task of the CPU how and what to
> put in RAM for the VIC-II to render it as graphics (actually the text
> does also works this way).
>
> - For the second, the VIC do read data from the RAM chips just for them
> to force their contents not to be lost.
>
> - For the third, the VIC do send a clock pulse from one of its pins to
> one of the pins of the CPU, that then outputs this signal to the rest of
> the system.
>
> So its inputs are power, of course, the clocks generated from the 8701
> or equivalent circuit, and the address and data bus.
>
> And its outputs are the video signals, the DRAM refresh lines, the IRQ
> lines, the BA line to control the CPU and stop it when needed.
>
> Then, there is the read/write pin and the chip select pin as control pins.
>
> So: what would be needed to emulate first of all for a C64 to believe
> that the VIC-II is still there?
>
> For me:
>
> - The clock output. The CPU, and the rest of the system, won't work
> without it.
> - The DRAM refresh lines. DRAM contents won't last without it.
> - The raster line register. There are things the CPU does when the
> raster line is in certain locations, like triggering IRQs.
> - The missing thing I am not aware of and that you are telling me.
>
> With an FPGA or a PIC fast enough, maybe there would be a way to
> implement this **as a first measure** to make a C64 *think* there is at
> a logical level a VIC-II there, because at an analog level, the system
> doesn't care about if there is a video signal coming from it or not, or
> if the graphics are actually rendered: the CPU just read or write
> registers, gets controlled by the BA line or IRQ, and gets ticked in by
> the clock pin.
>
> The next step would be reading from the proper places (ROM, RAM or COLOR
> RAM), and render the bitmap image taking care of the effects supposedly
> to be applied by the raster line tinkering that the demos do (of course
> sprites also have to be rendered).
>
> And finally, get that bitmap converted to an output signal that would be
> in a format that could be converted to *real* HDMI (digital) or
> component / svideo (analog) video. It could be even encoded inside
> network packets to have a VIC that could be remotely viewed through a
> network connection. This final stage could get rid, maybe, of the
> PAL/NTSC (and SECAM) limitations, because digitally it could be upscaled
> and up-frame-rated to modern standards, applying filtering - or just
> having a 4K version of the classic VIC graphics with the pixels upscaled
> to the size of actual bricks. ;)
>
> NOW is your turn (that I would actually appreciate as I love to learn)
>
> - What do you think?
> - Is it doable?
> - Does it worth the work to do it?
>
> Thanks all.
>
> --
>
> Saludos y cerveza.
>
>