Friday, April 3, 2015

C65GS at Revision 2015, and running on DDR

I still haven't fixed the DDR RAM controller.  However, I have found a work around to get the C65GS working on the DDR version of the board.  I am very relieved, because the DDR controller is still doing weird things, and I have yet to figure it out, which as readers of this blog will know has me very frustrated.

But, as I say, it is now possible to run the DDR board just fine, booting to C65 BASIC and everything.  This means I should be able to live with out a "Don't ask me about the DDR controller" T-Shirt.

The solution is that I am using almost every last spare bit of BRAM in the FPGA to have a 128KB "ROM" in the design, instead of storing the C65 ROM in DDR RAM. This has three main effects for now:

1. The "ROM" is now zero wait state, and as a result BASIC and the DOS routines fly. They are about 6x to 8x faster than they were previously.  The C65's horribly slow DOS routines can load somewhere around 50 - 100 blocks per second. You can also do FORI=1TO25000:NEXTI in about 1 second in C64 mode, i.e., around 50x faster than on a stock C64. In fact, BASIC is now so fast that you can use POKE to change the border colour about every 5 or 6 C64 raster lines.

2. I was planning to use the BRAM for enhanced sprites.  Clearly that can't happen now. I am thinking about how to feed sprites direct from DDR RAM, which would be fun.

3. The "ROM" is of course really RAM.  The Hypervisor can make it read-only for compatibility with any of the very few C65 programs that exist, if any happen to try writing over the ROM address space.  But for C65GS specific programs you can of course just use it as an extra 128KB of RAM.  I'll do another post about this soon, describing all of the RAM that is usefully available on the C65GS for programmers.

Play with a C65GS a Revision 2015!

Now, for anyone who happens to be in Saarbrücken for Revision 2015, a C65GS prototype will be their with deft.  He will be on the #revision IRC channel.  He has his FPGA board mounted in a C64 case with real C64 keyboard.  It is of course something that has been quickly pulled together, and a real C65GS would have a special PCB so that all the C64/C65 ports are available.  But it is still really nice to be able to interact with a C65GS with a real "body".  Hopefully deft will have Turbo Assembler and some other goodies on it (including a lame little demo I wrote for the C65 back in 1994) that you can try out if you would like.

2 comments:

  1. So, is deft at Revision? I can't track him on the IRC channel...

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    Replies
    1. Hello,
      Well, he was when I last spoke to him, but I haven't seen him on IRC there either -- just on skype. It may be that he is already leaving.
      Paul.

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