Re: NES on Apple II

[dsf@globalcrossing.net (Dan Foster)]

In article <1rSdnYQ7edYPXkSgXTWQlg@News.GigaNews.Com>,
Bryan Parkoff <BParkoff@satx.rr.com> wrote:
>
>    Is it possible to copy NES' ROM into Apple II memory location before I
>can be able to program using assembly language to emulate NES' ROM.  I

The Apple II is a computer consisting of a ROM, CPU, support chips, and
other stuff. If you take away any major component from that equation,
then it is no longer an Apple II. The GS was an exception because the newer
CPU still had a compatibility mode... and the expanded ROM still retained
the older ROM's key routines.

Apple II is just an unique identity made up of a specific combination
of different parts.

Are you still Bryan if I replace your brain with someone else's, but keep
all the other parts the same? That's what I mean by unique identity and
specific combination.

NES is basically roughly similiar, from a CPU perspective, but everything
else is totally different -- its memory map usage, its ROM, its support
chips, and other things.

Yes, technically you could replace the Apple II ROM with the NES ROM, but
you'd have to completely rewrite the NES ROM to work with the Apple II bus,
hardware devices, application support, and so on.

>    How many colors do NES support?  Why Apple II have only 6 colors?  Is it

The NES and the Apple II has completely different video controller hardware.
The NES uses a video controller known as the Picture Processing Unit (PPU).
The PPU can support up to 16 colors on the screen at the same time, but you
can, of course, dither the 16 colors to give an impression of much more
colors present.

The PPU's memory is separate from normal application RAM area, and can only
be accessed by touching special areas. *Completely* different from how the
Apple II does it, and very different limitations. The PPU is far more
powerful than the Apple II video controller, and has specialized functions
that it supports solely for game applications such as sprite handling.

Part of why the PPU is such so great for graphics-intensive "apps" is
because it offloads the main CPU of the task of keeping track of all these
pixels and making changes. Since Apple II was an outgrowth of an older
general-purpose computer developed in the mid-70s, it did not have the
benefit of the custom design specialized in a very specific task like the
PPU of the mid-80s.

Steve Wozniak was forced to deal with a limited number of colors originally
because it had to be a low cost general purpose computer, and he did not
have the budget for more elaborate parts or designs, and came up with a
number of nifty space-saving hacks... such as coming up with colors simply
by phase-shifting the video signal!

In about 10 years of mass market production, chip technology comes a very
long way in terms of size, capabilities, and cost. Nintendo also benefited
from their local expertise in chip design and fab plants. I think they
reverse engineered a copy of the 6502 processor -- and probably extended
it a bit, although I don't know for sure if they did or to what extent.

So, short answer to your original question is... no, you can't really
just drop a NES ROM in the Apple II ROM space and expect it to do anything
useful at all. In fact, it'd result in an unusable Apple II, and arguably,
it would no longer truly be an Apple II. The ROM is pretty much what gives
it the actual personality because it's a chip with all of the key behavior
information for various things -- booting, input, output, graphics, text,
printing, keyboard input, mouse input, disk hardware access, etc.

-Dan