> From: "Erik Mooney" <emooney@xxxxxxxxxxxxxxxx>
> I suppose you want zeropage, because if that wasn't a requirement, you
> could just use a ROM location. And I suppose you don't just want to
> dedicate one byte of RAM to the task of holding 0. And I'm not quite sure
> why an immediate LDA #00 wouldn't work. But here goes anyway:
Thanks Erik, just what I needed.
Your assumptions are essentially correct. I *HAD* one byte of RAM allocated
for this task (a guaranteed 0), but I figured this was wasteful. The code
below shows what I'm doing... I use x (the line index) to lookup the
address of the variable containing the data for a line. Some lines
("GUARANTEED0READ) require 0-byte data. With this setup (below), I don't
need to reserve a byte of precious RAM - provided I can find a location
(GUARANTEED0READ) which returns 0.
The "lda 0,y" loads the data from the correct data area for each line being
drawn (or, 0).
----------------------------------------------------
ldy TargetIndex,x ; 3
lda 0,y ; 4
sta TargetL ; 3
TargetIndex
; Points to the correct line in the TargetDisplay playfield
; Indexed by the scanline (0-18) being drawn.
; This allows the single kernel to draw the entire playfield (!)
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 0
blank
.byte TargetDisplay+0,TargetDisplay+0,TargetDisplay+0 ; 1
top row target
.byte TargetDisplay+0,TargetDisplay+0,TargetDisplay+0 ; 2
top row target
.byte TargetDisplay+1,TargetDisplay+1,TargetDisplay+1 ; 3
top middle row target
.byte TargetDisplay+1,TargetDisplay+1,TargetDisplay+1 ; 4
top middle row target
.byte TargetDisplay+2,TargetDisplay+2,TargetDisplay+2 ; 5
bottom middle row target
.byte TargetDisplay+2,TargetDisplay+2,TargetDisplay+2 ; 6
bottom middle row target
.byte TargetDisplay+3,TargetDisplay+3,TargetDisplay+3 ; 7
bottom row target
.byte TargetDisplay+3,TargetDisplay+3,TargetDisplay+3 ; 8
bottom row target
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 9
blank
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 10
"
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 11
"
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 12
"
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 13
"
.byte GUARANTEED0READ,GUARANTEED0READ,GUARANTEED0READ ; 14
"
.byte TargetDisplay+4,TargetDisplay+4,TargetDisplay+4 ; 15
cube sliding on bottom
.byte TargetDisplay+4,TargetDisplay+4,TargetDisplay+4 ; 16
"
.byte TargetDisplay+4,TargetDisplay+4,TargetDisplay+4 ; 17
"
.byte TargetDisplay+4,TargetDisplay+4,TargetDisplay+4 ; 18
"
----------------------------------------------------
I can put a CXCLR just before the ldy, so that'll do nicely.
Thanks again.
Cheers
A
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----- Original Message -----
From: "Erik Mooney" <emooney@xxxxxxxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Sunday, February 11, 2001 8:39 PM
Subject: Re: [stella] zero-read
> On Sun, 11 Feb 2001 11:47:57 +1100, you wrote:
>
> >I need a register location which, upon read, is ALWAYS guaranteed 0.
> >Can anybody suggest any/the best register or location to use?
> >Thanks
> >A
>
> I suppose you want zeropage, because if that wasn't a requirement, you
> could just use a ROM location. And I suppose you don't just want to
> dedicate one byte of RAM to the task of holding 0. And I'm not quite sure
> why an immediate LDA #00 wouldn't work. But here goes anyway:
>
> The collision registers map to $x0 through $x7, the paddles are at $x8 -
> $xB, and the joystick button inputs are at $xC and $xD, for all values of
> x 0-7. I _think_ $xE and $xF always return $FF, but I'm not sure.
>
> If you can do a CXCLR (clear all collision collision latches) closely
> before this routine so that you can ensure that no collisions of any one
> particular type will occur between the CXCLR and this routine, you can use
> that collision register. (example: $x4 contains the collision latches for
> missile 0 - playfield and missile 0 - ball. You can use $x4 to always
> return 0 upon read, if you can ensure that such a collision will not
> happen between a STA CXCLR instruction and this routine, which will be
> true if either you're not using missile 0, you're not using PF or ball, or
> you're in vblank.)
>
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