Subject: Re: [stella] Okay, newbie here. . . From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney) Date: Fri, 26 Feb 1999 16:06:55 GMT |
>??? I just discovered Stella programming a couple of weeks ago.? My >imagination was sparked because I've owned an Atari 2600 since I was . . >. about 6, I think.? I've always wanted to write games for it.? Anyway, Yep, that's about the way I got into it, too :) >I've downloaded DASM, compiled "How to draw a playfield," etc. and I >feel pretty good about my learning curve.? I'm picking up on this >"assembly" thing a lot faster than I did C++.? I have a few questions, >though. >??? 1.? What does the line "org $F000" do?? It's one of the few lines >that aren't commented. Within the memory map of the 2600's address space, the ROM area occupies the space from address $F000 to $FFFF. (that's 4k.) That directive just tells the assembler where to start the code within the 2600's address space. (the $ just means the following number is hexadecimal.) >??? 2.? What's the difference between a "register" and an "address?" Well, there's two different kinds of registers involved. The 6502 registers, internal within the microprocessor, are A, X, and Y (and the stack pointer and flags) - all they do is hold an 8-bit number. The Television Interface Adapter (TIA, aka Stella) also has a bunch of registers that are accessed via memory addresses $0000 through $002C . For all practical intents and purposes, you can just think of those registers as memory addresses that control screen graphics. Oh, and those registers are write-only - you can't read back from them. >??? 3.? In Nick Bensema's "How to draw a playfield," under "Vertical >Blank," it mentions machine cycles and includes the equation >37*76=2888.? Now, my calculator says 2812 every time I punch it in.? Was >this just a typo, or am I missing something? I think Nick typoed and did 38 * 76. -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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