Sinclair ZX81 User Manual page 49

3. Type in the program "CHARACTERS" again, & then type
LET X=7
so that - although it doesn't appear in the program - the computer now contains a variable X with value 7.
Now save the program, turn the computer off & on (to make sure there's no cheating), & load the program
back again. Type
PRINT X
& you will get the answer 7. The SAVE statement saved not only the program, but also all the variables -
including X.
If you want to keep these variables when you execute the program, you must remember to use GOTO &
not RUN (as was mentioned in chapter 9). You can avoid having to remeber this by making the program
execute itself (using SAVE as a program line).
4. Type in a very long program, & them momentarily disconnect the power supply. This sort of thing
sometimes happens spontaneously; it is not a bug, but a glitch. There is nothing you can do about it except
cry. It if happens more often than you can bear then there is probably something wrong, but it would be
worth saving the incomplete program on tape half-way through.