Anyone know hot to resolve this?(2d coordinates re

I have reproduced the error in a much smaller code to read!
The eror is the same...a sort of mirror around the center of the world axis (0,0)...

Type oggetto
Field posx
Field posy
End Type


Graphics 640,480,32,1
SetBuffer BackBuffer()
Global terreno=LoadImage("terreno.bmp") ;a simple texture of 256*256 
Global enemy=LoadImage("enemy.bmp")
MidHandle enemy
Global screenx=0
Global screeny=0
Global posx=200
Global posy=200
Global offset=1
While Not KeyHit(1)=True
Cls
x=MouseX()
y=MouseY()
mouse1=MouseHit(1)
Origin screenx,screeny
MapTiles()
;Origin 0,0
If KeyDown(200)=True Then screeny=screeny+1
If KeyDown(208)=True Then screeny=screeny-1
If KeyDown(205)=True Then screenx=screenx+1
If KeyDown(203)=True Then screenx=screenx-1

If KeyHit(57)=True Then screenx=0:screeny=0	;if i hit the spacebar

Rect MouseX(),MouseY(),9,9
;Origin screenx,screeny



If mouse1=True
create_enemy(screenx+MouseX(),screeny+MouseY())
EndIf






draw_enemy()
Origin 0,0

Color 255,255,255
Text 50,400,("Mouse X world coords:"+(screenx-MouseX()))
Text 50,410,("Mouse Y world coords:"+(screeny-MouseY()))
Color 255,0,0
Text 50,430,("X offset world coords:"+screenx)
Text 50,440,("Y offset world coords:"+screeny)
Color 0,0,255
Text 50,450,("X mouse world coords:"+(screenx+MouseX()))
Text 50,460,("Y mouse world coords:"+(screeny+MouseY()))



;If MouseHit(1)=True
	



Flip
Wend

Function MapTiles()
tilex=0
tiley=0
For tiley=0 To 2560 Step 256
For tilex=0 To 640 Step 256
DrawBlock terreno,tilex,tiley
Next
Next
End Function


Function draw_enemy1()
DrawImage enemy,posx,posy
posx=posx
posy=posy+offset
If posy =< 0 Then offset=1
If posy => 100 Then offset=-1
End Function



Function create_enemy(x,y)
a.oggetto=New oggetto
a\posx=x
a\posy=y
End Function

Function draw_enemy()
For a.oggetto = Each oggetto
DrawImage enemy,a\posx,a\posy
Next
End Function

If i understand this i can make the other program work...so as my game...