i think you need two processors for pic land
or infact any micros
one handles bcd the other operations
like display and addresses the calculator slave
via a psp port & a serial connection
this way you can break the psp into nibbles
and dump the calculated result accross a port in psp mode
dependant on the mode you set in serial
forinstance a basic stamp can have up to 5 programs reside
same can be done in a pic
http://www.geocities.com/andretuly/pv450addins.htm
i found this
it is a site with modifications
but containes other details and circuits also some links to other resorces
do you need a circuit of a proposed sc calc
or a keypad made in proteus for this purpose
if you do let me know and ill make you a keypad that works aok in proteus for a sc calc
get me a good picture of the front of one you want ill add also the keys names and any function letter
ill give you alsoa clear design to show the method