Manuel Polik wrote:
> Andrew Davie wrote:
>
> > > Can someone please find _and_ explain it to me? :-)
>
> > Well, this isn't an eor-trick. But fairly efficient.
> > Depends on what's your poison... code size or speed.
>
> Thanks for pointing out these two ways. The one with the table is the
> one I'll probably use, but the second one I liked more from a *style*
> point of view, since it's very clever that way.
Well, the most compact way is (as always) to count down (8 bytes):
dec FrameCounter
bpl .skipReset
lda #2
sta FrameCounter
.skipReset
But I don't know an EOR trick for this too.
> Just for the record, can someone come up with an elegant 'division by 3'
> solution?
> (quicker than subtracting multiple numbers of '3's I mean)
> That would save a BYTE'o'RAM then, since I wouldn't have to install
> another frame counter.
I guess, what you need is a framecounter that allows you to do something every 2nd, 3rd or maybe 6th frame. I had the same problem with Thrust too. My solution was to combine both counters into one byte. I used the upper two bits for counting to 3 and the lower 6 bits for counting up to 64.
That way I could simple define masks for the needed intervalls:
; masks for FrameCnt:
FRAME1MASK = %00000000
FRAME2MASK = %00000001
FRAME3MASK = %11000000
FRAME4MASK = %00000011
FRAME6MASK = FRAME2MASK|FRAME3MASK ; %11000001
FRAME8MASK = %00000111
FRAME12MASK = FRAME4MASK|FRAME3MASK ; %11000011
FRAME16MASK = %00001111
FRAME32MASK = %00011111
FRAME48MASK = FRAME16MASK|FRAME3MASK ; %11001111
FRAME64MASK = %00111111
FRAME96MASK = FRAME32MASK|FRAME3MASK ; %11011111
FRAME192MASK = FRAME64MASK|FRAME3MASK ; %11111111
And the code looked like this:
lda FrameCnt
and #mask
beq doSomething
The resulting code for increasing the framecounter was a bit complicated, but I needed one byte only:
lda FrameCnt ; 3
bpl .okCnt3 ; 2³
ora #$40 ; 2 reset Cnt3
.okCnt3:
clc ; 2
adc #$40 ; 2 increase Cnt3
sta FrameCnt ; 3
and #$3f ; 2
bne .okCnt64 ; 2³
lda #$3f ; 2 reset Cnt64
BIT_W ; 2
.okCnt64:
lda #$ff ; 2 increase Cnt64
clc ; 2
adc FrameCnt ; 3
sta FrameCnt ; 3 = 28-32
Have fun!
_______________________________________________________
Thomas Jentzsch | *** Every bit is sacred ! ***
tjentzsch at web dot de |
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