(rounded to nearest whole number) screensize/fontheight = screenlines
screenlines = how many lines of text the screen can hold
from = 0
to = from+screenlines
system(ls -F | nl)

upon keypress up:

if from-screenlines > 0 {

from = from-screenlines
{from++ to
print line from}

}

upon keypress down:

{

if from + screenlines < numberoflines {
from = from + screenlines
{ from++ to
print line from }
}

}