The 24-line screen
Timothy GiIbert,
Barry, South Glamorgano. 
Your Computer, February 1982, page 73

As an avid ZX-81 user I have discovered a few useful tricks while experiment-
ing. One of these is a 24-line screen. The screen is normally 22 lines by 32
columns, with another two lines at the bottom for input. Many probably know 
how to Poke characters on to these lower lines, but would it not be better to
Print or Print At on them? Here is how to.

The system variable at 16418 is the number of lines in the lower half of the 
screen. Although you are told it crashes the system if Poked, it does not in 
fact do so. By Poking in the value - the numbers on the screen are reversed in 
this variable - we are able to use the full 24 screen lines.

On these lines we can Print At - the bottom line is now 23 - and Print. This is
especially useful when adapting programs from systems with 24 screen lines and 
in producing a very large chess-type board. Do not try using Input or Scroll - 
you will crash the system.

As an extension to this idea, if you print what you require and then Poke 16418
with the number of lines you wish to protect, when you use Scroll only the top
half of the screen will move leaving your printed text untouched.

If you want to input something, Poke 16418,2 but ensure there is nothing impor-
tant on the bottom two lines as they will be cleared. I usually print my 
prompts on the normal screen only and protect them with a Poke. This leaves the
bottom two lines clear for my Input  subroutine, to obtain data without dis-
turbing my prompts but still leaving them protected on Return.

1000 LET RV=PEEK 16418
1010 POKE 16418,2
1020 INPUT R$
1030 LET RV=0
1090 RETURN