Re: A86: how does find pixel work?!?

[Prev][Next][Index][Thread]

Re: A86: how does find pixel work?!?




eightynine@juno.com wrote:

> Could someone please comment and explain whats happening here or tell me
> how to find out.  I am coming from the TI-85, not that it is too much
> different.  I have made some pretty cool games for the 85 using putsprite
> which uses find pixel, but I have never really known how it works.  I
> think that it is time that I learn.  This knowledge is what seperates
> people who know what they are doing from those who are just screwing
> around.  I want to be one of those who knows what is going on.  You can
> make plenty of cool stuff w/o knowing this crap, but like Matt Johnson
> says in his assembly tutorial having someone elses code in yours is a
> crappy feeling.  I should at least know how it works.   Thanks, Mike
>
> FIND_PIXEL:
>  push bc
>  push de
>  ld hl,ExpTable
>  ld d,0
>  ld a,b
>  and $07
>  ld e,a
>  add hl,de
>  ld e,(hl)
>  ld h,d
>  srl b
>  srl b
>  srl b
>  ld a,c
>  add a,a
>  add a,a
>  ld l,a
>  add hl,hl
>  add hl,hl
>  ld a,e
>  ld e,b
>  add hl,de
>  pop de
>  pop bc
>  ret
>
> ExpTable:
>  .db $80,$40,$20,$10,$08,$04,$02,$01
>
> _____________________________________________________________________
> You don't need to buy Internet access to use free Internet e-mail.
> Get completely free e-mail from Juno at http://www.juno.com
> Or call Juno at (800) 654-JUNO [654-5866]

 You can look on 86 central for a very commented findpixel routine.  It's not
so bad using other people's code if the code is super optimised.  But in that
case, use this find pixel routine:

;----------------------------------------------------------------------

; FIND PIXEL by CLEM  (117 cycles 34 bytes)
;
;  Input: x->b
;            y->c
;
;  Output: hl : byte in LCD memory
;               a  : bitmask for (hl)
;
; Destroyes : bc
;----------------------------------------------------------------------

FindPixel:
     ld h,63
     ld a,c
     add a,a
     add a,a
     ld l,a
     ld a,b
     rra
     add hl,hl
     rra
     add hl,hl
     rra
     or l
     ld l,a
     ld a,b
     and 7
     ld bc,FP_Bits
     add a,c
     ld c,a
     adc a,b
     sub c
     ld b,a
     ld a,(bc)
     ret

FP_Bits:     .db $80,$40,$20,$10,$08,$04,$02,$01
Follow-Ups: References: