A83: Subroutines

To: assembly-83@lists.ticalc.org
Subject: A83: Subroutines
From: benjamin99@juno.com (Alan C Johnson)
Date: Thu, 20 Aug 1998 15:01:46 -0500
Delivered-To: assembly-83-outgoing@towerguard.unix.edu.sollentuna.se
Reply-To: assembly-83@lists.ticalc.org


Hey guys, I was doing some thinking about the stack and I came up with
something.  Assumming you haven't loaded up the stack with data, could
you pop out of a subroutine.   This program waits for a key and then
quits.  It looks like a waste of space, but if your program is like the
one I'm making, it's a space saver.  This works because all running
programs is is calling them as subroutines from BASIC or whatever shell. 
When the program is called, the address to go back to when the program is
done is pushed to the stack, and retrieved by ret.  Basically call is
just this

	push	pc
	jp	Some_Label

and ret is

	pop	pc

So if you pop the return address from the call inside the program, it
skips over that call and and returns directly to the shell or OS's
address

Hope someone has a use for this info

Alan

BTW: Here's my example prog (Maybe you wanna use it James)

Getkey:
	call	_getkey
	cp	kEnter
	call	z,Quit
	jp	Getkey
Quit:
	pop	af
	ret

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