Atari XL

Advanced Techniques 197
before that most recently added. Try this example
program.
10 GOSUB 100
20
PRINT "MAIN
PROGRAM"
30 END
100
GOSUB 200
110 PRINT "SUBROUTINE 100"
120 RETURN
200
GOSUB 300
210
PRINT "SUBROUTINE
200"
220
RETURN
300
RETURN
If you run this program, the XL will follow the
GOSUBs to line 300 and then RErrURN
progressively through each subroutine, printing
the message. If you now change line 300 to read:
300 POP:POP:RETURN
and run the program again, the program will only
print (MAIN PROGRAM' and stop.
ERROR HANDLING
The XL has a command which allows you to take
corrective action should an error occur as a
program is running - TRAP.
The syntax is:
TRAP N
where N is the line number to which the program is
directed when an error is encountered. TRAP
must be executed before the error occurs, so you
should put it at the start of any program in which
you wish to use it.