Re: [stella] Don't say you weren't warned....

Subject: Re: [stella] Don't say you weren't warned....
From: Pete Holland <petehollandjr@xxxxxxxxx>
Date: Sun, 15 Aug 1999 11:07:51 -0700 (PDT)
--- Erik Mooney <emooney@xxxxxxxxxxxxxxxx> wrote:
> It starts at RAM location $FF (also accessible via
> $1FF, $20FF, $21FF, $40FF, $41FF, etc) and expands
> downward.

$FF is zero page and fastest, right?

> RAM is located from $80 to $FF, also accessible via
> $180 through $1FF, $2080 through $20FF, $2180 through
> $21FF, etc.

Same as above?

> (I know the Supercharger breaks most of what I just
> said, but let's not confuse him... :)

Thanks.  I'm confused enough as is. ;-)

> Right.  Feel free to look through some source code...
> my own INV includes a lot of this stuff; looking at
> the source  might help with the syntax.

Umm...where can I find this code of INV?  Is it at The Dig?  Is it
commented out, too?

> It counts time for you so your code doesn't have to
> do it.  By far the most common use is to, immediately
> after VSYNC, set TIM64T so that it will expire 37
> scanlines later.  Then you do all your calculation
> code, and then just watch the timer till it expires,
> then you know it's the right time to start drawing
> the screen.  This way, you don't have to make sure
> you catch every one of the 37 WSYNCs while doing your
> game calculation.

Just to make sure I'm clear on this:  would TIM64T be set for 37, or
the number of color clocks, or what?  And fits into the program how?  I
set it for whatever number, it automatically counts down and I check
for when it hits 0, then start drawing the screen?  Is TIM64T just a
number to check and I still need the WSYNC in the overscan period, or
is that handled by the command?  Also, would I store at TIM64T the
number for the overscan AFTER the frame, since it would be when that
expires that I VSYNC three times, then just reset the number for the 37
before the next frame?

> WSYNC is a strobe register; doing STA WSYNC doesn't
> actually shut anything on or off.  It's a signal to
> the TIA to keep the CPU in a halt state until the
> current scanline finishes.

So that makes it my responsibility to tell the program to switch it on
and off with a specific number that turns that bit on or off, right?

> The LDA sets the zero flag (that BNE tests) without
> need for a compare.

Whoa, no kidding.  Then which branch commands need a compare command
first, and which can just be done without it?

I'll get the hang of this yet....
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