Date: Thursday, April 13, 1995
From: jamesu@kelvin.mentorg.com
  Re: Re: Card slot #2 memory question!

I would guess that Program2 is the "unsafe" one because  the
GOSBVL =PUSH# may cause a garbage collection when making room
for the number to be pushed, causing the RTN to the Program2 
CODE object possibly to RTN to nowhere!  Looking at =PUSH#ALOOP,
we see essentially the same thing:

=PUSH#ALOOP
	R0=A
	GOSUBL  =PUSH#
	A=DAT0  A
	D0=D0+  5
	PC=(A)

with one important difference, the GOSUBL =PUSH# is called from ROM, which
can never move from its location due to a GC occurring.

Mika, I thought there was a different way, involving a GOVLNG =DOBINT
(or other data prolog), which would avoid GC when executing from RAM.

Finally note that GC may arise from pretty harmless looking calls and thus
the programmer may simply have overlooked it. Just compare the two programs
below and see if you have understood from above which one may cause a crash
and why (although the chances are small).


( Program1: negate # )		( Program 2: negate # )
CODE				CODE
	GOSBVL	=POP#			GOSBVL	=POP#
	GOSBVL	=SAVPTR 		GOSBVL	=SAVPTR
	A=-A	A			A=-A	A
	GOVLNG	=PUSH#ALOOP		R0=A
ENDCODE 				GOSBVL	=PUSH#
					LOOP
				ENDCODE