Commodore VIC-20 Programmer's Reference Manual page 92

may be typed but has absolutely no effect. The first four commands
shown can't be used because the same buffer that holds the
statement being executed is also used to hold the characters being
input. The DEF FN statement requires a line number so that the
formula may be referenced later
b. Program Mode
Instructions beyond a basic level of complexity require a
program, as opposed to direct mode commands which can perform j
simple commands in a single line of instructions. A program is one
or more lines, each having its own unique number and containing
a statement or group of statements. Line numbers must be whoie
numbers from 0 to 63999. Programs are usually written using every
10th number, or even 100th number, since most programmers
want to add more lines later between two existing lines, as the
program is developed or edited.
Line numbers are stored in numerical order regardless of the
order in which they are typed. The program will, when RUN,
execute from the lowest to highest numbered lines, unless there \s
a command to jump to a different Sine, like GOTO, IF ... THEN, or
GOSUB.
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