Atari XL User Manual

CHAPTER 9
LOGICAL THINKING
As well as doing arithmetic, the Atari XL can
perform tests to compare numbers and strings. We
have already seen this when using IF ... THEN. In
this chapter we will examine in more detail the
way in which the XL makes comparisons.
Consider the program line:
220 IF A = 3 THEN 500
The line instructs the Atari to branch to line 500 if
the variable A holds the value 3. What does the
Atari do when it encounters this program line?
The first thing the computer must do is decide
whether A is equal to 3; or to put it another way,
whether t.A = 3' is true. The XL tests this, and if
the expression is true, it returns the answer 1; if
the expression is false, the answer is O. If the result
is true (1), the commands after THEN are obeyed.
If the answer is false (0) the program will continue
with the next line.
Why are we telling you all this? Because the
computer can compare numbers and return true or
false values without IF. Try this short program:
10 A = 3
20
PRINT A = 3
The program displays t.1'. If you change line 10 to
70

Atari XL Specifications

General

Release Year	1983
CPU	MOS Technology 6502C
Graphics	ANTIC, GTIA
Sound	POKEY (4 channels)
Storage	Cassette tape
Ports	Cartridge
Operating System	Atari BASIC
CPU Speed	1.79 MHz
RAM	64 KB