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8bitworkshop/presets/vcs/setpos.inc
2018-08-27 16:50:53 -04:00

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603 B
PHP

; SetHorizPos - Sets the horizontal position of an object.
; The X register contains the index of the desired object:
; X=0: player 0
; X=1: player 1
; X=2: missile 0
; X=3: missile 1
; X=4: ball
; NOTE: This version of the routine does a NEWLINE before executing.
; It does NOT do a HMOVE and HCLR.
SetHorizPos subroutine
sec ; set carry flag
sta WSYNC ; start a new line
.DivideLoop
sbc #15 ; subtract 15
bcs .DivideLoop ; branch until negative
eor #7 ; calculate fine offset
asl
asl
asl
asl
sta HMP0,x ; set fine offset
sta RESP0,x ; fix coarse position
rts ; return to caller