Tuesday, August 23, 2016

Atari vs NES

It's really interesting to me the differences and similarities between Atari and NES programming. They both use the same 6502 processor (well, a variation on the same processor -- the Atari's is missing a few address lines, and the NES is missing a BCD mode), clocked at similar speeds (the nes is about 50% faster).

But beyond that, things start to diverge.  The most interesting to me is fundamentally where things happen, based on how their video chips work.

On the Atari, the video chip (the TIA) is designed to handle a scanline at a time. Which means that, like I've mentioned before, the whole time your TV is drawing, you spend that time babysitting the process, using all your processing time updating each scanline.  Then during vblank (the time the TV is repositioning the electron beam), you get to hurry and do your game logic updates.

On the NES, it's practically the opposite. The video chip (the PPU) is designed to handle a whole frame at a time (GASP!).  So during rendering, unless you're doing any fancy effects, you get all that time free to do your game logic updates, while the PPU does its thing in the background. Then during vblank, you hurry to push new data to the PPU so its ready for the next frame.

There's still hurrying involved in the NES -- there can be a lot of data that you want to shove into the PPU (new tiles for scrolling, etc), and not a lot of time to do it (most video data has to passed to the PPU over a janky serial interface, written one byte at a time), but compared to the Atari, I feel like I have EONS of time to work with.

It also helps having more RAM than I know what to do with. The Atari had 128 bytes. The NES has 2048 bytes. (Two whole kilobytes!)  It's crazy town.

There's still some challenges. Talking to the PPU is weird, you're limited to only 8 sprites per scanline, mid-screen scrolling (ie if you want to add a status bar) is tricky.  But so far, it's a different world.

Sunday, August 7, 2016

NES Game and LiveCoding

Well, I started messing around with making a NES game. I'm not sure I'm ready to tackle another big project right now, and there's still bits of work to be done with Atari Anguna (mostly fixing bugs as they get reported, then getting it ready for production), so I'm doing a little project:  sort of a simple 4-player space race game.

Tonight's tasks was writing the code to support the 4-player adapter, and scrolling the starfield.


The fun part is that I thought I'd try LiveCoding it, which is basically where you live stream yourself writing code and explaining it as you do.  I've tried this for the past 3 nights.  It's a really weird experience.

First, I don't know why anybody would want to watch someone code. I mean, at least watching someone stream video games is interesting because there's a lot of action. Coding is mostly thinking and typing.

Second, it's REALLY HARD.  Now I'm trying to do all my thinking out loud. Which is a really weird process.  Everything feels blurry and sluggish in my head. I feel like I'm not coding well, and not explaining well.  Maybe it gets easier, I don't know.  Then on top of that, trying to actually be somewhat entertaining while coding and talking?  Yeah right.

Third, I'm used to writing my hobby game code in the late evenings, as a way to sort of wind down. Only this doesn't wind me down, it winds me up. I've been finishing a bit after 11, and I'm all jittery and high-strung. There's just no way I can go to bed after spending an hour performing for strangers on the internet.  And I can't really do it sitting next to my wife on the couch as she watches TV, which is normally where the coding happens.

So I don't know how long this will last. On the one hand, it's tons of fun, because I have an audience. On the other, it's exhausting.  But hey, I might as well have fun with it for now.



PRGE 17

I just realized I never did a post after the Portland Retro Game Convention talking about how great it was! The first great part was just ...