alternative pset

[QbBasta]

screen 18,16

sub basic(byval memory as integer ptr,byval x as integer,byval y as integer,byval c as integer)
dim index as integer
index = y shl 9 + y shl 7 +x 'y * 640 + x
memory[index]=c
end sub

dim as integer ptr videomemory

screenlock:videomemory=screenptr

basic videomemory,10,10,rgb(25,0,25)
basic videomemory,638,478,rgb(25,0,25)

Why do i get a error when i wanna draw a pixel further down on the screen?
This works in 32 bit mode, but not in 16 bit?!