TO:  All programmers and users of MXO- series overlays

FROM:  Bob Abrahams, Los Angeles

DATE:  July 3, 1984

RE:  CLONING bug in some MEX overlays

In order to be properly "clonable", MEX overlays must meet
two requirements:

1) Any initialization must not "destroy" the ability for
the overlay initialization routine to be re-entered.  (This
is almost certainly okay in all working overlays.)

   - and -

2) Any settings in the overlay that can be changed during
MEX execution should be programmed so that the initializa-
tion section will set up the latest settings if cloned.
For example, if the baud rate default is 1200 bps in the
initial overlay, but the MEX program is cloned while set
to 300 bps, then the cloned version should initialize at
300 bps.  This fulfills the purposes of the CLONE
function.

Several overlays I've seen based on MDM7 versions do not 
do this properly, and a clone made at other than the 
original default setting will either still initialize at 
that original setting or work improperly.  For the baud
rate setting initialization, I recommend inserting a 
sequence to load the accumulator with MSPEED and calling 
the PBAUD (baud rate set from baud code) routine in your 
overlay.  Then the only indicator of the initial baud 
rate is MSPEED, and that will be changed by any sub-
sequent baud rate changes and will be cloned out.  Other 
initialization parameters that are changeable with later 
SET ccommands should use a similar strategy.