Re: [stella] Okay, newbie here. . .

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.

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