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NES 64: Commodore 64 KERNAL + BASIC port

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    calcwatch
    Posts: 1
    Joined: Tue Feb 15, 2022 6:07 am
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NES 64: Commodore 64 KERNAL + BASIC port

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Post by calcwatch >> Tue Feb 15, 2022 7:43 am

Hi everyone,

I'm new to these forums, but I've been messing around with 6502
assembly for decades...

A few years ago, I got the crazy idea to try porting the Commodore 64
KERNAL and BASIC ROMs to the NES, since their CPUs are mostly the
same. But I gave up when things got complicated with the PPU. Then a
couple weeks ago, I saw that someone ported it to the Atari 1200XL,
so I decided I'd give it another shot. This time, I was able to make
a (mostly) working system. :)

Here are some videos of it in action:

 1. Booting up, running BASIC, and showing tab stop and scrolling
    support: https://www.youtube.com/watch?v=G2APi8KVKGc
 2. Running a benchmark maze generator program: https://
    www.youtube.com/watch?v=ETx8FjbXT_0
 3. Running a benchmark that calls a machine language subroutine:
    https://www.youtube.com/watch?v=CrPb3mq1qwM
 4. Writing to the APU to play a C major scale: https://
    www.youtube.com/watch?v=KKzs_-WQxlI

I can't test it on hardware, but it seems fine in emulation. Since it
requires Family BASIC keyboard support, I think I'm limited to just
FCEUX and BizHawk, though I can't get BizHawk's keyboard support
working at all. FCEUX 2.6.2 seems a bit buggy too, so I had to write
some workarounds for it.

I'm also not sure about publishing the code for the project. Though
it seems that nobody cares about the various disassemblies already
floating around github...

Implementation Details
I went with the MMC5 mapper so that I could use ExRAM for the text
screen, and not worry as much about reading/writing bytes in the PPU.
The KERNAL an BASIC ROM are 8 kB each, so I put them in the last two
banks ($C000-$FFFF), and filled the other banks with PRG RAM
($6000-$BFFF). The first byte of OS RAM is reserved, and the rest is
available to BASIC. Hence the "24,575 bytes free" in the splash
screen: $C000 - $6001 = $5FFF = 24,575.

The CHR ROM just contains the default all-caps character set from the
C64.

I disabled all the mapper's interrupts except for the raster one,
which the OS uses for keyboard scans, etc.

Because it's using the MMC5, it gets an 8-bit-to-16-bit multiplier
for free. I tested it, and it works, but I can't find a practical use
for it. (Not that this project has any practically uses! :lol: )

The original code comes from a heavily annotated disassembly I found
on github. I needed to make some minor changes to get it assembled
with ca65, mostly adding colons after labels. To free up space for
new code, I removed pieces related to datacassettes, modems, RS232
devices and color RAM.

Caveats / Future Work
FCEUX only seems to support the shift key for letters. E.g.,
"shift+A" works, but "shift+1" doesn't register as a keypress. So
there's no way to type "!", or most other symbols you'd need for
BASIC. However, control key combos work, so I made the keyboard code
act as if control were the same as shift.

The C64 text screen is 40 columns by 25 rows. I changed it to 32x30,
which is fine for emulators, but I realize it's outside what TVs
would display. (Family BASIC's text screen is just 28x24.) Shrinking
it further would require substantial code changes to ignore the
screen RAM bytes before and after each screen line.

So that's all I have now. I hope you enjoyed the writeup! I still
can't believe that it works at all. :D
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    djmips
    Posts: 2
    Joined: Mon May 28, 2012 6:28 pm

Re: NES 64: Commodore 64 KERNAL + BASIC port

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Post by djmips >> Tue Feb 15, 2022 1:44 pm

Very fun. Thanks for sharing. How much further will you take this? Do
you have sprite examples?
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