You don't
Load it into an image instead, and then paste at negative y coordinates to 'scroll' it up - like this:
set window off
set display mode 800,600,32
create bitmap 1,800,1200
box 0,0,800,1200,rgb(255,255,255),rgb(255,0,0),rgb(0,255,0),rgb(0,0,255)
ink 0,0
center text 400,300,"This is the top half"
center text 400,900,"This is the bottom half"
get image 1,0,0,800,1200,1
sync rate 60
sync on
set current bitmap 0
do
for y=0 to 600 step 4
paste image 1,0,0-y
sync
next y
for y=596 to 4 step -4
paste image 1,0,0-y
sync
next y
loop