[stella] Phase Two

Subject: [stella] Phase Two
From: Pete Holland <petehollandjr@xxxxxxxxx>
Date: Sun, 21 Feb 1999 18:08:55 -0800 (PST)
Howdy, gang.  I think I have the general 6502 commands down.  I'm now
focusing on the Stella Programmer's Manual to see the specifics. 
Already, however, some questions occur to me.  I have downloaded DASM,
and when I finally get everything more or less together, I'm going to
try to program a solitaire game, figuring that would be a good "test"
to see if I grasp everything (or I'm I high?).  Before I do, I have
some questions.  Please don't laugh if these seem obvious.  I haven't
really taken an in-depth programming class.
1)  What is a "sprite"?  From the hype around my SMS, NES, Genesis,
and SNES, I was under the impression that a sprite is like a
Colorforms sticker, an image that the computer can move around over
the background.  If that's right, how can the Atari, which, from my
preliminary reading, needs to draw the entire screen, have sprites? 
Does it have sprites, or are they things that act like them?  How much
of this do I have wrong?
2)  I'm having trouble grasping the concept of cycle counting.  Is
this how it works?  The actual image the Atari draws is larger than
the TV screen.  The first forty lines don't appear, and while these
are being drawn out, since they only need a single color like black
and nothing pertinent to the game, this is where you have the CPU
doing calculations regarding the game (movements, calculations, etc.).
 The next 192 lines are where the picture is drawn, and since the
computer has to read the info, this is where you leave it alone to do
its job.  The next thirty lines are blank like the top forty, and more
game decisions can be made here.  How can you tell how much time you
have to do these things?  Do some commands take more time to execute
than others, and if they do, how long so that you can make it do the
calculations without disrupting the flow of the scan?
3)  Please explain the concept of MHZ to me.  From what I understand,
a hertz is a single cycle, so a megaHertz would be a million cycles. 
The TV screen, according to the Stella manual, draws the picture sixty
times a second.  That makes sixty hertz, right?  How do the "clock
counts" work with the hertz, then?  How many clock counts in a hertz?
Who knows?  With all these questions I'm asking, I may someday have
enough that I can write the tutorial people are dreaming about.
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