LATERAL SCROLL (16K ZX81)
by Darrin Hill, Saintbridge, Gloucester
from Your Computer (March 1982), Page 71

This machine-code program scrolls the screen from right to left. It will
only work on a 16K ZX-81 since it works by Poking on to the screen. The
main section scrolls one horizontal line, whilst the section beforehand
moves it down the screen until it has all been scrolled.

The asterisk on the right of the screen is moved up by "7" and down by "6".
You should find that it forms a tail due to the scrolling.

You do not have to scroll the whole screen. You can isolate it by Poking
certain locations.

    POKE 16530,A
    POKE 16531,B
    POKE 16515,C
    POKE 16516,D

A = 231, B = 0, C = 0 and D = 0 confine it to the top 7 lines. A = 214, B =
2, C = 231 and D = 0 confine it to the bottom 13 lines. A = 74, B= 1, C =
231 and D = 0 keep it in the centre and, so that the whole screen is
scrolled again, A = 214, B = 2, C = 0 and D = 0.

To decide on your own extremes Poke into 16530 and 16531 the hexadecimal
values of a number between 0 and 22 multiplied by 33 and adding 33 to the
answer. This sets the lower line. In all cases it must be greater than the
next value, which is a number between 0 and 22 multiplied by 33 and its
hexadecimal values Poked into 16516 and 16515.

Finally, since the program only scrolls from right to left it can be
altered so that it works in the opposite direction.

    POKE 16530,0
    POKE 16531,0
    POKE 16515,214
    POKE 16516,2
    POKE 16536,1
    POKE 16537,33
    POKE 16538,0
    POKE 16539,237
    POKE 16540,66
    POKE 16553,43
    POKE 16554,43
    POKE 16562,35