Atari XL

CHAPTER 17
ADVANCED TECHNIQUES
The previous chapters cover all you need to write
many programs for your XL. This chapter decribes
some further BASIC commands which you will find
useful as you wri te more ambi tious programs, and
rounds off wi th some general advice on how to
wri te good programs.
THE POP COMMAND
This strange looking command IS used In
connection with BASIC subroutines.
Suppose you have a program with subroutines
nested one within the other, one of which checks
which keys are being pressed. Y ou may wish to
respond to one of these keys - ESC perhaps - by
abandoning the current routine and returning to
the start of the program. This could mean
RETURNing through several levels of subroutine,
which would be tedious to program, requiring IF ...
THEN RETURN instructions at each level. The
XL allows a simpler way of doing this, using the
POP command
Each time a GOSUB command is executed, the XL
stores the location of the instruction after the
GOSUB on a stack in the reserved area of memory.
At any time the stack contains RETURN
addresses for all GOSUBs for which RETURN
commands have not yet been performed. The POP
command removes the most recently added
RETURN address from the stack, so that the next
RETURN sends the computer back to the GOSUB
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