Erik Mooney wrote:
>
> >(...)
> >>LF0B2: LDA $B2,X ;4
> >> STA NUSIZ0 ;3
> >> STA REFP0 ;3
NUSIZ uses bits 5, 4, 2, 1, and 0, but not bit 3. REFP0 uses bit 3
only, so you can store both values in one byte with a simple ORA. That
was probably done earlier, and the values are being fetched and used
above.
> >> LDA VSYNC ;3
> >> LSR A ;2
> >> LSR A ;2
> >> ORA VBLANK ;3
> >> STA $D9,X ;4
Get collision status for Missile 0 vs. Player 0 and 1 into bits 7 and
6, shift them to over to bits 5 and 4, then get collision status for
Missile 1 vs. Player 0 and 1. One byte now contains all 4 different
collision values of Missiles vs. Players.
> >(...)
> >
> >I bet this has something to do with random numbers...
No, I don't think so.
> That is strange.. VSYNC is a write-only register; its register 0, so LDA
> VSYNC reads from register 0, which is CXM0P (a collision register). VBLANK
> is also a write-only register (register 3) which corresponds to read
> register CXP1FB, which is another collision register.. the ORA would have
> to read from register 3, which would be CXP1FB. I don't know what's
> supposed to be going on here.
VBLANK equ 1, which is CXM1P when read.
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