--- Erik Mooney <emooney@xxxxxxxxxxxxxxxx> wrote:
> We love novices! All tender and chewy... oops,
> was that out loud? :)
Tender and chewy, yeah, but probably way above your cholestarol count.
:-)
> BEQ looks a certain bit in the flags register (called
> the zero flag), and branches (jumps) if that bit is
> set. Just about every mathematical operation, as
> well as load and transfer instructions, either sets
> or clears the zero flag depending on its results.
> The operand provided to BEQ is the offset to the
> address to jump to if the branch condition (zero flag
> set) is met.
So by even taking a number from, say, $FCCC and copying it to the X
register, that will set the zero flag?
The above indicates to me that, before I get to the really important
decision parts of the program, things that affect the line count and
variables, it would be good to clear all carry flags so this way any
changes directly related to the game set the flags and I'm not taking
chances on what's up or down, right?
> So if your code is
> LDA #2
> STA $85
> CMP $85 ;compares the contents of A with the
> contents of $85; it's equal
> BEQ label1
> DEX
> label1: [more code]
>
> since the CMP resulted in equality (which sets the
> zero flag), the BEQ will branch to label1.
Is it possible to compare, say, $FCCC and $F281 directly without
transfering numbers to the X and Y registers?
Does BEQ branch just one command ahead, or is there a way to set it to
branch about three commands ahead?
> I think both are correct in a way. There is the
> VSYNC period, which lasts three scanlines, during
> which you can do about 80-100 commands. Then there
> is the VBLANK period, during which you can do about
> 800-1000 commands.
Got it. By the way, the programming manual says that writing to the
VSYNC register initiates the action. Is just the act of writing to it
what triggers the VSYNC, or does it have to be a different byte than
what is in VSYNC at that time so it can tell the difference? And if it
has to be a different number, can I just INC or DEC that register to
produce the results?
> Yep! You can do whatever you want during any
> scanline.
Great. But what color is produced by VBLANK? Is it black since the TV
doesn't show it anyway, or is it whatever the background color is set
to at that moment? I guess I'm trying to figure out what color to make
the background if I make a Columns game and use the extra VBLANKs to
check for more matches.
Does VBLANK work like VSYNC? E.G. STA VSYNC and STA VBLANK?
> You're getting closer, yeah, but sloooow down,
> grasshopper! :)
Grasshopper? Do I have to shave my head AGAIN? ;-)
Sincerely,
Pete Holland Jr.
"A little army/We were commandos
"Guerillas with decoder rings."
The Aquabats
-"Super Rad"
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