Could you refer to the following program?
ACLS
SPSET 0,0
POSX=200:POSY=120
WHILE TRUE
VSYNC 1:STICK OUT SX,SY
DX=SX*(ABS(SX)>=0.125)
DY=-SY*(ABS(SY)>=0.125)
POSX=POSX+4*DX
POSY=POSY+4*DY
SPOFS 0,POSX,POSY
WEND
another way is to do
ACLS
X=0:Y=0
While 1
VSYNC
STICK OUT SX,SY
INC X,SX*2
INC Y,-SY*2 '(*2 MAKES SPEED FASTER BTW)
SPSET 0,0
SPOFS 0,X,Y
WEND
this should work unless I forgot something