HIGH RES for Sinclair ZX-81
by M Geerling and S Besmon
from Your Computer, December 1984

When we saw a high-resolution game in Your Computer we thought:
"Let us write our own high-resolution program". This was not such
an easy task. We came up with a program which offers high-res programs
in Basic.

Our program is stored above Ramtop which enables you to re-write your
existing programs. Including the high-resolution display-file and a
character-set it takes approximately 7,000 bytes.

The machine-code program transforms the normal display file into the
high-resolution display file.

The code of a character in the normal display file corresponds with the
eight codes of a UDG - User-Definable Graphic.

When the Basic program is run you are asked to define UDGs, or not. If
the answer is no then the program will be stored above Ramtop including
the character-set. If no UDGs were defined before, this set is empty. If
the answer is yes the computer will ask for a character-code. Thereafter
it will ask for the eight numbers to build up a UDG.

When you have completed the UDG-defining you have two options, either you
can save the character-set including the complete program on tape, or - if
you do not want to save on tape - it is all stored automatically above
Ramtop.

When the program is stored above Ramtop, you can write your own program or
load an existing one. A program should begin with:

    RAND USR 25614

to put the computer in high-resolution mode. You can transform from normal
display-file by:

    RAND USR 25686

As CHR$(0) is not transformed you should print a CHR$(1) to remove a
character from the high-resolution display - the transforming routine
removes CHR$(1) from the normal display file. You should finish with:

    RAND USR 25602

to put the computer back to normal resolution.

The high-resolution screen is cleared by:

    RAND USR 25626

The character-set starts at address 16717 - this is in Ramtop: 25772. Do not
use the ZX-81 Scroll, because the computer will crash.
