GdosTool - By J Davis 08/1990

Why
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An  easy  mistake  to make when progrmming in assembler is to accidentally
use  $0 (the value in memory location $0) where you mean #$0 (the constant
value  zero).   A  subtle  mistake - one not aided by the fact that memory
loaction  0  often holds 0, so it will yield the right results.  

The first you notice of this problem is when your code is run on some a590
equipped  500's  ,  or  2000s with 2091 controllers, where $0 may actually
hold  the  string  'GDOS'.  The same situation can also occur in most high
level languages, by de-referencing a NIL pointer.

The  result  can  be anything from small errors upto a full blown GURU (if
the  code  tries  to  de-ref  $0  as a pointer to a word - it will yield a
non-word aligned word access)
 
This is a common problem (other programs that have trouble with a non zero
location  0  are  virusx4,  lharc1.20,  execellence  1.11,  Jrcomm 1.0 and
several others).
 
By  using  this tool you can set location zero to a 'problem' value before
testing  your code - this will hopefully make the release of programs with
such 'mistakes' less common.
 
You  can  also  set  it  back  to  $0000000 - useful after you've finished
testing, and also potentially useful for people with a590s or 2091s.