Atari XL User Manual

40
The Atari XL Handbook
Atari not to go on to the following lines, which
contain the subroutines.
The subroutine at line 110 inputs two numbers and
stores them as A and B. The subroutine at line 210
clears the screen and prints the two numbers and
their sum.
When the program is RUN, the computer begins,
as always, at the lowest numbered line, line 10.
This line calls the first subroutine, and the
computer is diverted to line 110. Line 130 returns
the computer to the instruction after the GOSUB,
which is in line 20. The program proceeds as
normal to line 30, which causes another diversion
to line 210. Line 240 returns the computer to line
40, and the program ends.
So, a GOSUB command causes a diversion from
the sequence of a program to a subroutine, and the
RETURN command ends the diversion.
The use of subroutines saves a lot of effort in
writing programs, as the program lines in the
subroutine need to be written only once, instead of
being retyped at every point in the program where
they are needed. There is no limit to the number of
times a subroutine may be called.
Another advantage of subroutines is that they can
make the design of a program simpler. If you use
subroutines for the repetitive and less important
parts of a program the main program becomes
much easier for you, the programmer, to follow,
and is therefore much easier to write.
Subroutines, like loops, may be nested - that is, the
subroutines may call other subroutines, which may
in turn call others. However, a subroutine may not
be called by another subroutine which it has itself
called, or an endless loop occurs. The program will

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